android: Replace deprecated onBackPressed() and enable predictive back gestures

Doesn't really make a difference it seems.

src/frontends/android/app/src/main/AndroidManifest.xml

@@ -37,6 +37,7 @@
         android:label="@string/app_name"
         android:theme="@style/ApplicationTheme"
         android:networkSecurityConfig="@xml/network_security_config"
+        android:enableOnBackInvokedCallback="true"
         android:allowBackup="false" >
         <activity
             android:name=".ui.MainActivity"