Introduce audio_hal_version_class flag for BluetoothAudioHalVersion

Bug: 315241296
Bug: 307595549
Test: m libbt-audio-hal-interface
Change-Id: I2f5457d3dd77a9362551ce89c6e904de893ba816

flags/hal.aconfig

+package: "com.android.bluetooth.flags"
+
+flag {
+    name: "audio_hal_version_class"
+    namespace: "bluetooth"
+    description: "Change bluetooth audio hal version from enum to class"
+    bug: "315983648"
+    is_fixed_read_only: true
+}