Disable analytics by default on debug builds

Use the supported manifest metadata tags to disable Analytics and
Crashlytics by default for debug builds. This uses a manifest
placeholder to toggle the setting between debug and release builds.

This depends on endlessm/python-for-android#18.

.p4a

@@ -33,3 +33,7 @@
 --depend com.google.firebase:firebase-crashlytics-ndk:18.3.2
 --depend com.google.firebase:firebase-analytics:21.2.0
 --with-debug-symbols
+--debug-manifest-placeholders '[analytics_enabled: "false"]'
+--release-manifest-placeholders '[analytics_enabled: "true"]'
+--meta-data firebase_analytics_collection_enabled=${analytics_enabled}
+--meta-data firebase_crashlytics_collection_enabled=${analytics_enabled}