Implement IBufferClient::close on BufferClient

The close() function will mark a BufferClient as closed (set mClosed =
true), remove it from service's mClientList, and remove all of its token
in service's mTokenMap.

Add test cases to make sure it works in all situations.

Test: BufferHubBuffer_test (passed)
Bug: 119623209
Change-Id: Ic1d17ced97b67ef5432c9d341469d8e6105e2717