Setting the default value for dock audio

There was no default value for parameter "dock_audio_media_enabled"
in global settings and because of that it couldn't be obtained before
Settings app first start and improper actions, such as FORCE_NONE
sending to AudioSystem instead of FORCE_ANALOG_DOCK, were taken.
This patch sets default value to 'true'.

Change-Id: Idbe343519db15f806f3a237e8b39f8420b9edde1
Signed-off-by: Dmytro Dubovyk <dmytro.dubovyk@ti.com>