Remove mFocusedStack from ActivityManagerService.

Use ActivityStackSupervisor.mMainStack in its place. Or, in cases
where multiple stacks must replace mFocusedStack, create new
methods in ActivityStackSupervisor that will replace it.

Change-Id: Ib0dda51e1043a6d5110e5a2914d695ef16bf9371
3 files changed