Settings: Reduce volume to safe level when headset is connected (1/2)

This is a safety feature for users who opt to raise volume above the designated
safe level while using a headset.

Currently when a user does this they are provided with no protection against
dangerously high volume levels until the next time the headset volume warning
is displayed.

This patch adds an optional feature to ensure volume is no higher than the safe
level every time a headset is connected.

Change-Id: I8ec35284a91a26511b0bc83fcc1578f38b8b4f40