[APM]: only boost a11y volume during ringtone

This condition was removed with change
e763f7ad. Also make sure that the index
used when computing the volume on the ring curves is rescaled.

Test: manual (instructions in bug)
Bug: 439876768
Flag: EXEMPT bugfix
Change-Id: Ic25963a5174013a3f5c4e38975b42082cb3d74d4