Fixed permission issue with AM.setFocusedStack/Task APIs

- Check for MANAGE_ACTIVITY_STACKS permission when AM.setFocusedStack
or AM.setFocusedTask is called.
- Clear calling identity when AM.setFocusedStack is called.

Bug: 27135250
Change-Id: Ib082bc6cd03411c4cbf741e5bdcec54679441eff
1 file changed