Merge "document that media_codecs_sw.xml is part of mainline" am: b383ac72 am: 7c9d6f97

Original change: https://android-review.googlesource.com/c/platform/frameworks/av/+/2247581



Change-Id: Ic4ea47470411d147b8bee8274197566cb552361c
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>