Generate Buffer classes with GensrcBuffer.gmk

The .java files can now be re-generated by
ojluni/src/tools/gensrc_android.sh.

New APIs will be added in the future CLs, but now they
are @hide until the relevant tests are imported.

Android patches in .java has been ported into the .template files.

http://b/305287555 has details why BufferMismatch is patched here.

Bug: 269740550
Bug: 271236407
Bug: 305287555
Test: m droid
Change-Id: I2e076d0b0a5d3c3c83ee48c67a131035add1635d
31 files changed