Fix IME action logic on CVC field in CardMultilineWidget (#790)

**Summary**
If the postal code field is not shown, the CVC field
is the last one in the form and the action on the
keyboard when the CVC field is focused should be
"Done" (i.e. checkmark). Otherwise, show the "Next"
action (i.e. right arrow).

**Motivation**
Showing the correct IME action improves user experience
and accessibility

https://developer.android.com/training/keyboard-input/style#Action

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

@@ -10,6 +10,7 @@
 import android.text.TextWatcher;
 import android.util.AttributeSet;
 import android.view.View;
+import android.view.inputmethod.EditorInfo;
 import android.widget.LinearLayout;
 
 import androidx.annotation.ColorInt;
@@ -245,24 +246,30 @@ public void setEnabled(boolean enabled) {
 
     void adjustViewForPostalCodeAttribute() {
         // Set the label/hint to the shorter value if we have three things in a row.
-        @StringRes int expiryLabel = mShouldShowPostalCode
+        @StringRes final int expiryLabel = mShouldShowPostalCode
                 ? R.string.expiry_label_short
                 : R.string.acc_label_expiry_date;
         mExpiryTextInputLayout.setHint(getResources().getString(expiryLabel));
 
-        @IdRes int focusForward = mShouldShowPostalCode
+        @IdRes final int focusForward = mShouldShowPostalCode
                 ? R.id.et_add_source_postal_ml
                 : NO_ID;
         mCvcEditText.setNextFocusForwardId(focusForward);
         mCvcEditText.setNextFocusDownId(focusForward);
 
-        int visibility = mShouldShowPostalCode ? View.VISIBLE : View.GONE;
-        mPostalInputLayout.setVisibility(visibility);
+        final int postalCodeVisibility = mShouldShowPostalCode ? View.VISIBLE : View.GONE;
+        mPostalInputLayout.setVisibility(postalCodeVisibility);
 
-        int marginPixels = mShouldShowPostalCode
+        // If the postal code field is not shown, the CVC field is the last one in the form and the
+        // action on the keyboard when the CVC field is focused should be "Done". Otherwise, show
+        // the "Next" action.
+        mCvcEditText.setImeOptions(postalCodeVisibility == View.GONE ?
+                EditorInfo.IME_ACTION_DONE : EditorInfo.IME_ACTION_NEXT);
+
+        final int marginPixels = mShouldShowPostalCode
                 ? getResources().getDimensionPixelSize(R.dimen.add_card_expiry_middle_margin)
                 : 0;
-        LinearLayout.LayoutParams linearParams =
+        final LinearLayout.LayoutParams linearParams =
                 (LinearLayout.LayoutParams) mCvcTextInputLayout.getLayoutParams();
         linearParams.setMargins(0, 0, marginPixels, 0);
         if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN_MR1) {