SystemServer: Access adapter with lock

Accessing the mAdapter should be done while holding a readLock to
prevent concurrent access.
In the future, the lock should no longer be necessary as all access to
the mAdapter will be done on the correct thread

Test: m service-bluetooth | no op change
Bug: 288450479
Change-Id: I77888bd85a7d1e1c9a346d2ea1b9fb5fef9f82a8