Add RootTargetPreparer for explicitly specifying the root dependency am:...

Add RootTargetPreparer for explicitly specifying the root dependency am: fb283ce5 am: 8b1efdc4 am: cfbb9d94 am: 2ae19061 am: 18c1440c am: 9ac30506

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



Change-Id: I147da3174bd879841ccd6483b484c9e02bf4a9a5
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>