Add androidx support

[Jeff11]

We get Unresolved reference because android.support.v7.app.AppCompatActivity is not recognized when using androidx.

[sockeqwe]

Unfortunately we are in a bad situation with all this androidx package renaming.
I can't update Mosby to androidx without breaking other apps.

However, what you can do is write your own class BaseActvitiy extends androidx.appcompat.app.AppCompatActivity and then just use the Mosby internal deleage.

Basically you just have to copy & paste the Mosby internal classes
https://github.com/sockeqwe/mosby/blob/master/mvp/src/main/java/com/hannesdorfmann/mosby3/mvp/MvpActivity.java

which actually then just delegates / forwards lifecycle events to the corresponding delegate.