Protect id generator with GUARDED_BY am: 59e302b89e am: edfacb8c7c

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/native/+/23562843

Change-Id: Iaff480a6605069fdc31ec7d4df96e0ca8f6d4883
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>