Add RootTargetPreparer for explicitly specifying the root dependency am: fb283ce556 am: 8b1efdc4e2

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/26681387

Change-Id: Ia607b7986113b0d761ee65392972a9904ba6cc81
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>