Handle focused tasks on multiple displays

When the focused task changes displays, make sure to mark the whole
previous display as not part of a focused task.

This change also adds an adb am command to change the task focus.

Test: android.server.cts.ActivityManagerDisplayTests
Test: #testStackFocusSwitchOnTouchEvent
Bug: 35214007
Change-Id: I9cb7372c21a0b592abb6f6d910077ff5097dd6cf