See build/soong/README.md for more information.
Also moves a few headers into the same library as their
implementation, and fixes a few includes to go through the
global frameworks/av/include path instead of manually adding
frameworks/av/include/drm.
Test: m -j checkbuild
Change-Id: I0c2d2f2262e7ed78d80a0d44795705c64b797ff0
