Prevent AddSourceActivity's SourceCallback from being GCed (#791)

**Summary**
`Stripe#CreateSourceTask` holds a `WeakReference` to the
`SourceCallback`. This reference was being garbage collected,
causing callback (either onSuccess or onError) to never happen.

Making the SourceCallback an instance variable seems to fix
this issue.

**Motivation**
Because the callback was never happening, the user would be
stuck on the "Add a Card" screen.

**Testing**
Tested manually on various API levels.

stripe/src/main/java/com/stripe/android/SourceCallback.java

@@ -1,5 +1,7 @@
 package com.stripe.android;
 
+import androidx.annotation.NonNull;
+
 import com.stripe.android.model.Source;
 
 /**
@@ -12,11 +14,11 @@ public interface SourceCallback {
      * Error callback method.
      * @param error the error that occurred.
      */
-    void onError(Exception error);
+    void onError(@NonNull Exception error);
 
     /**
      * Success callback method.
      * @param source the {@link Source} that was found or created.
      */
-    void onSuccess(Source source);
+    void onSuccess(@NonNull Source source);
 }

stripe/src/main/java/com/stripe/android/Stripe.java

@@ -763,7 +763,8 @@ private void validateKey(@NonNull @Size(min = 1) String publishableKey) {
         }
     }
 
-    private void executeTask(Executor executor, AsyncTask<Void, Void, ResponseWrapper> task) {
+    private void executeTask(@Nullable Executor executor,
+                             @NonNull AsyncTask<Void, Void, ResponseWrapper> task) {
         if (executor != null) {
             task.executeOnExecutor(executor);
         } else {
@@ -772,23 +773,23 @@ private void executeTask(Executor executor, AsyncTask<Void, Void, ResponseWrappe
     }
 
     private static class ResponseWrapper {
-        final Source source;
-        final Token token;
-        final Exception error;
+        @Nullable final Source source;
+        @Nullable final Token token;
+        @Nullable final Exception error;
 
-        private ResponseWrapper(Token token) {
+        private ResponseWrapper(@Nullable Token token) {
             this.token = token;
             this.source = null;
             this.error = null;
         }
 
-        private ResponseWrapper(Source source) {
+        private ResponseWrapper(@Nullable Source source) {
             this.source = source;
             this.error = null;
             this.token = null;
         }
 
-        private ResponseWrapper(Exception error) {
+        private ResponseWrapper(@NonNull Exception error) {
             this.error = error;
             this.source = null;
             this.token = null;
@@ -850,7 +851,7 @@ protected ResponseWrapper doInBackground(Void... params) {
         }
 
         @Override
-        protected void onPostExecute(ResponseWrapper responseWrapper) {
+        protected void onPostExecute(@NonNull ResponseWrapper responseWrapper) {
             final SourceCallback sourceCallback = mSourceCallbackRef.get();
             if (sourceCallback != null) {
                 if (responseWrapper.source != null) {

stripe/src/main/java/com/stripe/android/view/AddSourceActivity.java

@@ -34,15 +34,16 @@
 public class AddSourceActivity extends StripeActivity {
 
     public static final String EXTRA_NEW_SOURCE = "new_source";
+
     static final String ADD_SOURCE_ACTIVITY = "AddSourceActivity";
     static final String EXTRA_SHOW_ZIP = "show_zip";
     static final String EXTRA_PROXY_DELAY = "proxy_delay";
     static final String EXTRA_UPDATE_CUSTOMER = "update_customer";
 
-    CardMultilineWidget mCardMultilineWidget;
-    CustomerSessionProxy mCustomerSessionProxy;
-    FrameLayout mErrorLayout;
-    StripeProvider mStripeProvider;
+    @Nullable private CardMultilineWidget mCardMultilineWidget;
+    @Nullable private CustomerSessionProxy mCustomerSessionProxy;
+    private FrameLayout mErrorLayout;
+    @Nullable private StripeProvider mStripeProvider;
 
     private boolean mStartedFromPaymentSession;
     private boolean mUpdatesCustomer;
@@ -63,6 +64,24 @@ public boolean onEditorAction(TextView v, int actionId, KeyEvent event) {
                 }
             };
 
+    @NonNull private final SourceCallback mSourceCallback = new SourceCallback() {
+        @Override
+        public void onError(@NonNull Exception error) {
+            setCommunicatingProgress(false);
+            // This error is independent of the CustomerSession, so
+            // we have to surface it here.
+            showError(error.getLocalizedMessage());
+        }
+
+        @Override
+        public void onSuccess(@NonNull Source source) {
+            if (mUpdatesCustomer) {
+                attachCardToCustomer(source);
+            } else {
+                finishWithSource(source);
+            }
+        }
+    };
 
     /**
      * Create an {@link Intent} to start a {@link AddSourceActivity}.
@@ -136,28 +155,12 @@ protected void onActionSave() {
         final SourceParams sourceParams = SourceParams.createCardParams(card);
         setCommunicatingProgress(true);
 
-        stripe.createSource(sourceParams, new SourceCallback() {
-            @Override
-            public void onError(Exception error) {
-                setCommunicatingProgress(false);
-                // This error is independent of the CustomerSession, so
-                // we have to surface it here.
-                showError(error.getLocalizedMessage());
-            }
+        stripe.createSource(sourceParams, mSourceCallback);
 
-            @Override
-            public void onSuccess(Source source) {
-                if (mUpdatesCustomer) {
-                    attachCardToCustomer(source);
-                } else {
-                    finishWithSource(source);
-                }
-            }
-        });
     }
 
-    private void attachCardToCustomer(StripePaymentSource source) {
-        CustomerSession.SourceRetrievalListener listener =
+    private void attachCardToCustomer(@NonNull final StripePaymentSource source) {
+        final CustomerSession.SourceRetrievalListener listener =
                 new CustomerSession.SourceRetrievalListener() {
                     @Override
                     public void onSourceRetrieved(@NonNull Source source) {
@@ -250,8 +253,9 @@ interface StripeProvider {
     }
 
     interface CustomerSessionProxy {
-        void addProductUsageTokenIfValid(String token);
+        void addProductUsageTokenIfValid(@NonNull String token);
 
-        void addCustomerSource(String sourceId, CustomerSession.SourceRetrievalListener listener);
+        void addCustomerSource(String sourceId,
+                               @NonNull CustomerSession.SourceRetrievalListener listener);
     }
 }