Fixed issue with empty sample gamAppId

[joaocsousa]

1. Fixed issue where if the sample_gam_app_id is empty, it correctly defaults to the hardcoded sample gam app id.

If the sample_gam_app_id is in the gradle.properties but empty (as it is when you clone for the first time), the app simply crashes because orElse will simply return an empty string.

2. Fixed issue whereby we were setting the manifest placeholder with an instance of a Provider<String> instead of its value.

providers.gradleProperty("sample_gam_app_id").orElse("ca-app-pub-3940256099942544~3347511713") returns an instance of Provider<String> instead of its value as String. So effectively an instance of Provider<String> was being put in the manifestPlaceholders map, which doesn't sound right.