Add mainline extension and METADATA.txt to boot.zip. am: dac9d66a

Original change: https://googleplex-android-review.googlesource.com/c/platform/build/+/23658853



Change-Id: I7ae8685e93742016f4267377df1c82db41a66948
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>