Sound Control: fix applying wrong values on boot

When rebooting, DSP Manager restores the RAW values instead of the
ones, with the offsets applied.
This causes the values to be terrible high.

We prevent this issue with applying the offset when writing the
value instead of applying it when saving the value as preference.

Change-Id: Ib935a041cde0a3fdd0280f1fa41ffd13eac8fee4