TextToSpeech memory leak

The callback object needs to be out of the class so that it won't
get a strong reference on the TextToSpeech object because it leads
to a memory leak due to the fact that the callback's lifecycle is
linked to the server and not to the TextToSpeech and then, keeps it
from being GCed.

Change-Id: I9cd5c7726d7cf2f60d6e25cc5caa87ad825e8f83
Author: Jean-Christophe PINCE <jean-christophe.pince@intel.com>
Signed-off-by: Jean-Christophe PINCE <jean-christophe.pince@intel.com>
Signed-off-by: Shuo Gao <shuo.gao@intel.com>
Signed-off-by: Bruce Beare <bruce.j.beare@intel.com>
Signed-off-by: Jack Ren <jack.ren@intel.com>
Author-tracking-BZ: 57869