BT: Use static lock to avoid race while cleanup

Problem:
The functions protected by mutex lock are called after cleanup
during shutdown. During this race condition, the structure set does not
exist and cause crash.

Steps:
Bluetooth on/off automation

Failure:
Bluetooth process crashes

RootCause:
Mutex is accessed after it is destroyed because of other thread calling
the function.

Fix:
Changed mutex lock to static type so that it exist and protect
the set structure even after cleanup.

Test: manual

Bug: 66921327

Change-Id: I8d1f5d5d607100ab73aec722a27688dae45cc323