[sf] make legacy frontend debug flag persistent am: 3e76a50f am: 3af3282e

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/native/+/21801174



Change-Id: I1132475031c59da42b091f5667171319456f8f7a
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>