LeAudio: Fix volume control

The calculation formula is now set to be the inverse of the one used
in VolumeControlService.java when calculating volume index from the
BLE audio volume level.

This also fixes not using the absolute volume for BLE audio stream,
which could result with very low volume levels at the lower registers
of the volume slider. It was caused by lowering the volume on both
ends. Not transmitting the sound at the maximum volume level could
also reduce the sound quality due to effective sound sample resolution
being lower.

Bug: 238587620
Bug: 241501978
Test: built and manually tested
Tag: #feature
Change-Id: I368c8978be5fbd94b0f62df2551fe179a9f203a0