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

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



Change-Id: I8d44852f371b68294d102f7e1678e9c5361f405c
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>