Report top resumed activity state change

Some system resources can only be accessed exclusively by a single
client at a time. Previously there was only one resumed activity in
the system at any point, so apps were usually trying to obtain such
access in onResume. With multi-resume this is no longer a good place
to do so, because:
- onResume may be called for activity that is not on top;
- top resumed status may be switched between different resumed
  activities in multi-window mode by tapping on them.

This adds a separate callback to inform apps about top resumed state
gain and loss events. The following rules apply:
- both callbacks can only be delivered between onResume and onPause;
- top resumed state loss always follows top resumed state gain;
- the callbacks are optional, activity can go from onResume to
  onPause without obtaining top position.

Bug: 117135575
Test: atest CtsActivityManagerDeviceTestCases:ActivityLifecyclePositionTests
Change-Id: I45797afe47b166b22cfb75b64132b22d3fb4ecc7