audioservice: modify default music volume

Default music volume is changed from 3/4 of max to 1/3 of max to
reduce initial volume over headphones percieved as too agressive.

Also add a property ro.config.media_vol_default to modify this
default value.

Bug: 62608451
Test: verify default music volume over headphones after factory reset.
Change-Id: Icadb99e1bafe88984d3a3a05014d4d4648830693