Enable predictive back gestures

Our activities are so simple, there's nothing special needed to support
this.

Signed-off-by: Andrew Gunnerson <accounts+github@chiller3.com>

app/src/main/AndroidManifest.xml

@@ -21,11 +21,13 @@
         android:backupAgent=".backup.ConfigBackupAgent"
         android:icon="@mipmap/ic_launcher"
         android:label="@string/app_name"
+        android:enableOnBackInvokedCallback="true"
         android:memtagMode="off"
         android:networkSecurityConfig="@xml/network_security_config"
         android:roundIcon="@mipmap/ic_launcher_round"
         android:supportsRtl="true"
-        android:theme="@style/Theme.RSAF">
+        android:theme="@style/Theme.RSAF"
+        tools:targetApi="tiramisu">
         <meta-data android:name="com.google.android.backup.api_key"
             android:value="unused" />