Add CarAudioFocusEnforcementTest and refactor utils

Introduces a new parameterized CTS test to verify the audio focus
silencing policy across different audio usages.

Key changes:
  - Adds CarAudioFocusEnforcementTest to ensure apps with
    managed audio usages are correctly silenced.
  - Refactors the usageToString method into a new CarAudioUtils
    class for better code reuse.

Bug: 409332426
Flag: android.car.feature.audio_focus_enforcement
Test: atest CarAudioFocusEnforcementTest
Change-Id: Id827d20c37a3af6ac0b3b75c1a15e946e96aa6d5