Improvements to SF's BackgroundExcecutor.

- stop using utils/Singleton.h (it's marked as DO NOT USE)
- add a low priority instance
- explicitly set the thread priorities for both the HP and LP instances.

Test: manual, libsurfaceflinger_unittest
Bug: http://b/336461947
Change-Id: Ie10f663ed43aad21ba2a5235cd86a1101dec0cd7