Add mainline extension and METADATA.txt to boot.zip. am: dac9d66a am: 0f096fd3

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



Change-Id: Id3240b43779b9799c9c5e209313ef6e1ea1b8e80
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>