[Media] Update AudioManager and AudioFocusRequest API docs with changes for Android 15.

Noted that if an app targets Android 15, it can't request audio focus unless it's on
top or running a foreground service.

Staged:

https://android.devsite.corp.google.com/reference/android/media/AudioManager
https://android.devsite.corp.google.com/reference/android/media/AudioFocusRequest
https://android.devsite.corp.google.com/reference/kotlin/android/media/AudioManager
https://android.devsite.corp.google.com/reference/kotlin/android/media/AudioFocusRequest


No change to code, docs-only fix.

Flag: DOCS_ONLY
Test: [ABTD docs build]
FIXES: 411696483
Change-Id: I2b2eaf7aa0db9a698e4fa1ed14fed3b905e1fcc8