Add WindowManager side timeout for toasts.

If the thread a toast is shown on is shut down,
the attempt to post the hide message to it's handler
will fail and it will never request removal. If this was
only some application background thread we will also not
receive a death notification. It seems best to use a timeout
to ensure we don't need the clients cooperation, espescially
as toasts can keep the screen on.

Bug: 21693547
Change-Id: I1d6e54ded5b9e2050daedc4d263e2e21fbe69862