Remove default impls from clock plugins am: 24ec252394 am: 7cad34e015

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/23438120

Change-Id: I9d9608b1c47fd0feb40eb540f4a47ddbe63b0d35
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>