Set the AVRCP absolute volume control initial volume to 50% for all devices

* Introduced a new configuration value named
  "a2dp_absolute_volume_initial_threshold_percent" that can be used to
  specify the absolute volume control initial value as a percent of
  the maximum volume level.

  Valid values are in the [0, 100] interval, and the default value is 50%.

* Marked the existing configuration "a2dp_absolute_volume_initial_threshold"
  as @deprecated.

Bug: 65754094
Test: Manual test with devices that have different numbers of volume steps
Change-Id: I449c5de52c68e66a3c01721bf6324f3090cfc33c