multiple_maps - Android: Use pure Android X and latest Google Maps SDK

Change Imports for Android Support Library to Android X
Remove Plugin dependency cordova-androidx-build from plugin.xml. This is not needed anymore.
Raise android-minSdkVersion from 19 to 21. This was needed for this PR.
Remove com.android.support:multidex:1.0.3 from pgm-custom.gradle. This was needed, when the minSdkVersion was lower then 21.
Use original Google Maps SDK dependencies again and not any longer a deprecated and decommissioned beta version
Google Maps SDK dependencies versions are configurable again with GOOGLE_MAPS_PLAY_SERVICES_VERSION and GOOGLE_MAPS_PLAY_SERVICES_LOCATION_VERSION
Cleanup of pgm-custom.gradle

mapsplugin / cordova-plugin-googlemaps