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 am: a92592d6

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



Change-Id: Ib30f3f60f0b07cb7b7079076f1518d3373ddfe0b
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>