Protect id generator with GUARDED_BY am: 59e302b89e

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

Change-Id: Icafb77bc452b8271a80ac1ffef0e94d8c48885cf
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>