Add timeout to StrictMode flash.

Consider an app that requests that a StrictMode border be shown, and
then immediately crashes.  We'd end up showing the bars indefinitely,
which is a terrible UX.

This CL enqueues a second message to tear down the bars if nobody
asks for them to be taken down within 1 second.

Test: manual app that crashed after showing violation bars
Bug: 37787765
Change-Id: I45d2078d0dcaa94e8f24087d028f5f06d12c4349