Android: Update minimum required version

Since Qt 5.12 is an LTS, Qt 5.13 is a good place to increase the minimum Android version supported by Qt. Increasing the minimum version will allow us to use more modern Android APIs and hence to support more modern features in the OS.

Current options are either Android 19 (which currently gives us around 98% coverage): https://developer.android.com/about/dashboards/

Or Android 21, which would bring us to a more recent major update and still give us around 90% coverage.