Enable HCI Sniff mode and disable subrating for A2DP

Enabling the HCI Sniff mode could help headsets to save power.

Note that subrating is disabled implicitly by NOT sending the
"Sent Sniff Subrating" HCI command to the BT controller,
and effectively using the default Sniff Subrating parameters.
The Headset shouldn't go into Subrating Mode to prevent
potential stuttering if the Headset chooses to use subrating
with the wrong parameters.

NOTE: This should affect only Pixel Buds.

Also:
 - Added some extra log messages
 - Minor cleanup (argument/variable renaming)

Bug: 74344711
Bug: 74361002
Test: Manual - audio streaming; examine btsnoop logs
Change-Id: Iec5086dba6f9003c21bcb41eddb3bc83ac7dddd1