Add RootTargetPreparer for explicitly specifying the root dependency am: fb283ce556

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

Change-Id: I437f62992d011621f69ac56e6689acd2ed3f7c7b
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>