[SF] Extract {Display|Layer}RenderArea to separate files

This CL also fixes incorrect sampling bounds in RegionSamplingThread,
caused by ag/11837111.

Bug: 158599281
Test: atest libsurfaceflinger_unittest
Test: take a screenshot from launcher
Test: take a screenshot from YouTube while playing a video
Change-Id: I7e7777071e91a833a7bad4bd77e9d0c989fea9b7