android: Fix restart on resizing or splitting screen

This fixes an issue where the app would restart on entering split
screen, or on resizing the app's side of a split screen.

Basically this line of the Android manifest is making an assurance to
the system that the app is prepared to handle these changes; docs here:
  https://developer.android.com/guide/topics/manifest/activity-element#config
When the system doesn't get that assurance, it just restarts the app
from scratch to be sure.

And empirically the app indeed does just fine when we add those
assurances and then go ahead and switch to split-screen mode, or
resize the app's portion of the split screen.  That layer of things is
basically all RN's job, so RN is handling this just fine -- we just
haven't been letting the system know that we're ready to do so.

Also got this fixed in the RN template app upstream:
  facebook/react-native#32536
  facebook/react-native@fc962c9b6

[greg: expanded commit message]

Fixes: zulip#5068

android/app/src/main/AndroidManifest.xml

@@ -70,7 +70,7 @@
         android:theme="@style/AppTheme">
         <activity
             android:name=".MainActivity"
-            android:configChanges="keyboard|keyboardHidden|orientation|screenSize|uiMode"
+            android:configChanges="keyboard|keyboardHidden|orientation|screenLayout|screenSize|smallestScreenSize|uiMode"
             android:label="@string/app_name"
             android:launchMode="singleTask"
             android:windowSoftInputMode="adjustResize">