Bluetooth: Restrict gain for Absolute volume case

For the lowest music volume steps 1 and 2, restrict the gain to 50% and
75%. This will avoid the lowest volume steps being too loud for some
accessories. For music volume 0, set phone gain to 0 as some
accessories won't mute on their end.

Change-Id: I24e0fa7be8c8635b428a11c91ea153aad7cec55f
Signed-off-by: Liejun Tao <baibai@motorola.com>