SF: Permit changing DispSync offsets at runtime

This modifies EventThread such that its phase offsets for both
Choreographer and SurfaceFlinger may be modified at runtime. It also
plumbs this functionality up to the SurfaceFlinger debug interface so
that it is possible to rapidly test different offsets without
restarting the framework.

Change-Id: I426873f8553f931250dfebc9a8a4a78e9f1f4309