Fix change of behavior in Looper.quit().

It seems some applications rely on Looper.quit() terminating the
loop immediately without processing all messages.  Rather than
risk breaking them, make the safer behavior optional.

Also take care to properly drain the message queue before quitting
so that all of the Message instances are recycled.  This may
help release storage sooner in case the Looper doesn't get GC'd
promptly and its remaining queue of undelivered messages sticks
around.

Improve docs on runWithScissors.

Bug: 8596303
Change-Id: I8cbeb6f7a5f6b8e618b5109f87a03defc1486b9f