Add RootTargetPreparer for explicitly specifying the root dependency am: 58077070

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



Change-Id: I49b9f504f5b314be4d49ad266cbf365ddef8cbb8
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>