android: Set launchMode="singleTask" instead of "singleTop"

This addresses zulip#620 by making it so the whole app state isn't reset
when coming back from Firefox in the web-auth flow. Discussion:
  https://chat.zulip.org/#narrow/stream/48-mobile/topic/Flutter.20app.20.2B.20SAML.20auth/near/1776828

Later in that discussion, Greg found that this mode is the most
appropriate for our app, and that setting taskAffinity="" is
probably a good idea as a security-hardening measure.

Fixes: zulip#620

android/app/src/main/AndroidManifest.xml

@@ -7,7 +7,8 @@
         <activity
             android:name=".MainActivity"
             android:exported="true"
-            android:launchMode="singleTop"
+            android:launchMode="singleTask"
+            android:taskAffinity=""
             android:theme="@style/LaunchTheme"
             android:configChanges="orientation|keyboardHidden|keyboard|screenSize|smallestScreenSize|locale|layoutDirection|fontScale|screenLayout|density|uiMode"
             android:networkSecurityConfig="@xml/network_security_config"