Add RootTargetPreparer for explicitly specifying the root dependency am: 3c4f5273ea am: e866baa800

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/26764622

Change-Id: Id5afcde742093746849bea66c938dc4ff4e79a8a
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>