Add RootTargetPreparer for explicitly specifying the root dependency am: 58077070 am: 8682e50f

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/26928402



Change-Id: I4a7c02a4daf96168dcefff9542fa2d9172d02d40
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>