Remove stopped activity from UnknownAppVisibilityController.

UnknownAppVisibilityController tracks activities whose visibility are
not yet settled, and delays app transition. When activity is stopped,
it doesn't appear on the screen anymoere, so we shouuld remove it from
UnknownAppVisibilityController.

Bug: 191392295
Test: manual
  1. go to lock screen
  2. tap calender
  3. check if the calender app starts within a couple of seconds.
Change-Id: I2c99469d44ee6c1e3ccd7b7b915321ecce0900ce