Hearing Aid volume curve

Hearing Aid uses volume value from -127 to 0dB. Make sure that volume
curve used is able to adjust volume in whole range.

Test: play music using Hearing Aid, try lowering volume to smallest
non-mute value.
Bug: 69623109
Fixes: 76434849
Change-Id: I8b8d64405016252f08ddda823fab74115352de4e