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: 9b84bba1 am: 82d28994 am: 5d518598

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



Change-Id: Ica246ac0266656eefe16455d7b169d8b6c73e3b3
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>