Add RootTargetPreparer for explicitly specifying the root dependency am: 6d6cd37cd7

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/native/+/26789419

Change-Id: Ibd85585b3f4a1957a50fcabca46fe50d35ddb1d4
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>