Avoid NPE when sendMessage is called after disconnect.

This avoids an NPE that could occur when:
1. disconnect() is called
2. sendMessage() is called but encounters a RemoteException
3. replyDisconnected() will attempt to dereference mSrcHandler
There does not appear to be any callers that rely on the NPE.
All callers erase their reference to the AsyncChannel after
calling disconnect(), except for NetworkAgentInfo which can
cause ConnectivityService to crash.  This fix addresses that.

bug:20647016
Change-Id: I89864885dc3371941407a036b7b7647e0ec037b8