Cleanup AudioManager volume flags

Rename "Flags" IntDef to "PublicVolumeFlags" to express what
 those flags are for.
Align volume flag definitions with the API type (public SDK vs
 SystemApi). Because there are many flags, list them all and
 list which ones are omitted, and state why.
Remove "one or more flags" parameter comment as it is confusing
 with the generated IntDef comment which correctly indicates that
 you can use the value 0 when not passing any flags.

Bug: 176476452
Test: check generated javadoc for android.media.AudioManager
Change-Id: Id5c86aec922a06d468375e9cfa9ff58387ef3b33