fix a race condition of MediaPlayerServce::Client::mClient

there will be race condition between mClient.clear in MediaPlayerServce::Client::disconnect method
and c=client->mClient in in MediaPlayerServce::Client::notify method

Change-Id: I767294c4cb864c08e9c0e9f23d6d304a39016205
Signed-off-by: beanz <beanz@marvell.com>