LeAudio: Fix volume issues

This fixes not setting the absolute volume mode for the BLE Audio use
cases. It also adjusts the volume calculation from BLE volume level
to volume index to match the same formula used in BTHelper.java
for the reverse calculations (one can be easily derived from the other).

Since BLE Audio device has more granular volume control, If an earbud
changes its volume by a tiny step, multiple BLE volume values will be
translated into the same volume index and it may look like the volume
slider didn't move. And on the other hand if the audio FW changes the
volume by a whole index (or by 10 indices at once) the BLE device volume
will increment by a much bigger step and the volume difference will be
noticable on each step.

Bug: 238587620
Bug: 241501978
Test: atest BluetoothInstrumentationTests --no-bazel-mode
Tag: #feature
Change-Id: I54a73080b84bb9cbb3e2d799aff9fe7d5fa71e84