[sf] make legacy frontend debug flag persistent am: 3e76a50f am: 9d4abc96

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



Change-Id: Icbb6b5d8551af938ed9b8d9fc94eb113bb4fe0c1
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>