Fix the bug where inner lock in MultiRateLimiter is null.

The bug would occur when Mockito stopped mocking methods when the test
is done, but the mocked object still exists, and is then accessed as a
scheduled message triggers it.

Test: atest NotificationManagerServiceTest
Bug: 174997218
Change-Id: I3c76456927bb0c27945b137187011b5eb3c7102c