Fix ANR caused by hwuiTask thread

If hwuiTask thread is exited while HWUI renders something,
some tasks can remain unfinished forever.
This can make ANR problem if RenderThread waits this kind of tasks.

According to the current implementation, hwuiTask threads are
exited when HWUI receives trimMemory() callback with level >= 20
and some applications such as SystemUI can receive trimMemory()
with level >= 20 even though they renders something yet.
(For instance, when RecentsActivity in SystemUI is finished,
HWUI receives trimMemory() callback with level >= 20
but SystemUI should still render the status bar and navigation bar.)

This patch prevents the tasks from remaining unfinished and
make the tasks executed immediately if they cannot be added
to their TaskProcessors.

Change-Id: I5bd26439aa5f183b1a7c1ce466362e27554b4d16