AudioVolumeGroup: implement hashCode method

Implement Object.hashCode() method.
Allow AudioVolumeGroup.getAudioVolumeGroups() to be called
 by holders of MODIFY_AUDIO_SETTINGS_PRIVILEGED, like the
 other volume group APIs.

Bug: 340879593
Test: atest AudioManagerTest#testVolumeGroupHashCode
Change-Id: Ie4898d013eef9352bb5b9fa7ce7e09fca095356e