android: Override MainActivity.invokeDefaultOnBackPressed, following …

…Expo

This belatedly follows a change in Expo's template app,
templates/expo-template-bare-minimum. The change was made in
expo/expo@2a0079132, and we missed it in the Expo 44 upgrade (zulip#5441)
because Expo's upgrade guide forgot about it. See discussion of this
and other changes that were missed:
  https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/Expo.2044.20small.20things/near/1452850

This seems to fix a bug I've observed on the office Android device
(Samsung Galaxy S9 running Android 9), where if I exit the app via
the back button, then re-enter the app, the state gets reset such
that we register for a new event queue.

android/app/src/main/java/com/zulipmobile/MainActivity.kt

@@ -1,5 +1,6 @@
 package com.zulipmobile
 
+import android.os.Build
 import android.content.Intent
 import android.os.Bundle
 import android.webkit.WebView
@@ -91,4 +92,23 @@ open class MainActivity : ReactActivity() {
         }
         super.onNewIntent(intent)
     }
+
+    /**
+     * Align the back button behavior with Android S
+     * where moving root activities to background instead of finishing activities.
+     * @see <a href="https://developer.android.com/reference/android/app/Activity#onBackPressed()">onBackPressed</a>
+     */
+    override fun invokeDefaultOnBackPressed() {
+        if (Build.VERSION.SDK_INT <= Build.VERSION_CODES.R) {
+            if (!moveTaskToBack(false)) {
+                // For non-root activities, use the default implementation to finish them.
+                super.invokeDefaultOnBackPressed()
+            }
+            return
+        }
+
+        // Use the default back button implementation on Android S
+        // because it's doing more than {@link Activity#moveTaskToBack} in fact.
+        super.invokeDefaultOnBackPressed()
+    }
 }