Remove usage of abs_volume_index_fix

This flag has been rolled out since 24Q3 and can be removed.

Test: atest VolumeHelperTest, AudioDeviceVolumeManagerTest
Bug: 340693050
Flag: EXEMPT removing com.android.media.audio.abs_volume_index_fix
Change-Id: Ia1fd7e11350e42e6b0a87f944c03ad141d0a1144