Prepare for SDK v20

[manishram]

There are many breaking changes coming in version 20.0.0. Version 19.7.0 introduced many new APIs, and deprecated or renamed many classes in preparation for version 20.0.0.

https://developers.google.com/admob/android/migration

#539
#560

[mikehardy]

https://github.com/invertase/react-native-google-mobile-ads supports v20