android: Handle "Safer component exporting" change on targeting Andro…

…id 12

See the change described at
  https://developer.android.com/about/versions/12/behavior-changes-12#exported .

A value of "true" means that the component accepts input from other
apps.

- The launcher is another app, so we need "true" there so that Zulip
  can be launched.

- We need "true" for the share-to-Zulip activity; the content to be
  shared originates from other apps.

android/app/src/main/AndroidManifest.xml

@@ -70,6 +70,7 @@
         android:theme="@style/AppTheme">
         <activity
             android:name=".MainActivity"
+            android:exported="true"
             android:configChanges="keyboard|keyboardHidden|orientation|screenLayout|screenSize|smallestScreenSize|uiMode"
             android:label="@string/app_name"
             android:launchMode="singleTask"
@@ -92,6 +93,7 @@
 
         <activity
             android:name=".ShareToZulipActivity"
+            android:exported="true"
             android:label="@string/app_name"
             android:launchMode="standard"
         >