Add garbage collecion function to BufferHubService

Now BufferHubService will cleanup unused BufferClient automatically.

If the mClientList storing weak pointer, when user side's strong pointer
dies, the refcount will become 0 and the allocated memory in the service
will get freed.

Test: BufferHubBuffer_test (passed)
Bug: 119623209
Change-Id: I1390f878bf44afd7c1a5cccaeb34a9d13dab3db0