Add new mode for SCHED_FIFO on UI and RenderThreads.

Add a new mode, controlled by sys.use_fifo_ui property, that enables the
top app's UI and RenderThread to be SCHED_FIFO. This eliminates almost
all jank due to scheduling competition with non-UI critical
threads. This mode may not be suitable for all devices.

bug 24503801

Change-Id: I7b8a31830ad80f7efa00236928d5476998ed4e00