catch mutex when notify sampling rate changed, otherwise signal may be lost...

catch mutex when notify sampling rate changed, otherwise signal may be lost am: e1e361ed am: ab8e9f1b

Original change: https://android-review.googlesource.com/c/platform/hardware/interfaces/+/2496635



Change-Id: I5a0567009ee8e9c6ae792888ea9992f6d2ea1442
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>