Stop P2P test cases in onStop() instead of onPause()

P2P dialogs will be implemented in a separate Activity, which causes the
test Activity to be paused in the background and stops the test. To
prevent this, the tests should be stopped in onStop() (not visible to
the user) instead of in onPause() (visible to the user).

Bug: 209032090
Test: CtsVerifier Wi-Fi Direct tests
Change-Id: I0f51b042de750aef01f6cff22b34ea217adfa69c