Plumbing through GPU context priority

If the SF's RE priority is set to REALTIME, then SysUI set's it's
ThreadedRendererCompat priority to EGL_CONTEXT_PRIORITY_HIGH_IMG.

Test: Compile and observe logcat.
Bug: 168740533
Change-Id: Ib2fdac90c22b357d8b9f5c80c03a3d9c4e1a515d