SurfaceFlinger: Add EventControlThread

This change adds a new thread for calling HWComposer's eventControl
asynchronously.  The DispSync-based vsync approach ends up enabling and
disabling HWComposer's vsync callbacks at arbitrary times, and some HWComposer
implementations do not have these calls optimized.

Bug: 11175503
Change-Id: I719be82bd200b391c61d40863b991c7b59acdfd6