Enable predictive back gestures

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

Signed-off-by: Andrew Gunnerson <[email protected]>

app/src/main/AndroidManifest.xml

@@ -43,6 +43,7 @@
         android:allowBackup="true"
         android:icon="@mipmap/ic_launcher"
         android:label="@string/app_name"
+        android:enableOnBackInvokedCallback="true"
         android:memtagMode="sync"
         android:roundIcon="@mipmap/ic_launcher_round"
         android:supportsRtl="true"