mediaserver: conditionally launch ListenService

Conditionally instantiate ListenService from MediaService.
ListenServiceruns as a singleton in its own thread that calls
AudioHAL which in turn accesses Listen Driver. It is the entry
point from Application processes that want so execute Listen
commands and receive Voice detection events. It manages Listen
application resources and global states across all Listen
applications.

ListenService is similar to AudioFlinger. That is why it is
instantiated from MediaServer. Listenservice talks to AudioHAL
to deal with concurrent usage of mic.

Change-Id: I71c346bb16e0ed3a7aae9c908e03c7f1c694fe0a