Separate MediaRecorderClient with MediaPlayerClient

- An alternative would be to define a common base class that
  both MediaRecorderClient and MediaPlayerClient can derive.
  But since the common code, onTransact() and notify() uses
  some Binder code, having a common base class may not gain
  us too much in terms of code reuse.

Change-Id: Ibc06720278ad173fceacff3d267b7060856c6316