Glimpse: Set android:enableOnBackInvokedCallback

We don't intercept back press events, so we can simply opt-in

Change-Id: I3e6e925239ab8bfe16d3af58563c821a66d8ba75
diff --git a/app/src/main/AndroidManifest.xml b/app/src/main/AndroidManifest.xml
index 6584301..4e90df1 100644
--- a/app/src/main/AndroidManifest.xml
+++ b/app/src/main/AndroidManifest.xml
@@ -19,6 +19,7 @@
 
     <application
         android:name="org.lineageos.glimpse.GlimpseApplication"
+        android:enableOnBackInvokedCallback="true"
         android:icon="@mipmap/ic_launcher"
         android:label="@string/app_name"
         android:supportsRtl="true"