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: I72dadcbcf507391b422990286269b54b1bb9d54b
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>