Make task fullscreen when locked

Previously if we were in multi-window mode and some task called
startLockTask() or kiosk mode was started - other tasks remained
on screen. User could still interact with other tasks, locked or
pinned task could be removed.
This CL makes tasks move to fullscreen when they are locked.

Bug: 27876860
Change-Id: I547a94be50276c7f418343600877e51b6b40c7a7