DnD: Total silence doesn't suppress A11y

When entering Total silence mode, text prompts from a11y were losing
 AppOpsManager.OP_PLAY_AUDIO, causing them to play muted (amplification
 of 0.0f) even though the STREAM_ACCESSIBILITY volume was not.
The fix consists in adding another category of sound suppression
 behavior in AudioAttributes, for usage types that should never be
 suppressed (== muted), and using it for USAGE_ACCESSIBILITY.
 When ZenModeHelper iterates over usages to mute/unmute players,
 consider whether that usage's suppression behavior is SUPPRESSIBLE_NEVER.

Test: turn Talkback on, then enter Total silence and verify Talkback is heard
Bug 62827456

Change-Id: I48cae48797ef9bc6bcaee82484ba078ee445345c