Toast timeout: Just strip FLAG_KEEP_SCREEN_ON

We added the toast timeout to fix b/21693547. Where the client
leaks non visible toasts which end up keeping the screen on.
Simply removing the toast windows as we were doing previously,
isn't very good though. If the timeout fires when an app could
still use the toast (e.g. due to long pause under debugger),
the app could crash. It's not obvious how (if even possible
without API semantic/guarantee change) to prevent this issue
from the client side. Furthermore, #33 at b/21693457 implies that
we need to respect FLAG_KEEP_SCREEN_ON even for windows that
never submitted a buffer. With time constraints as they are,
the only solution I can think of that solves both without
chance of regression is just stripping FLAG_KEEP_SCREEN_ON when the
timeout fires.

Bug: 29105388
Change-Id: Ic2df897f86acf814ea3d4f911a0532d65af0c19f