Handle wired headset during voicemail playback.

- Switch to wired headset when plugged in even if playing on speaker
- Remember if wired headset overrode speaker so it can be turned back on
  when the headset is unplugged.
- Ensure proximity sensor is turned on iff the earpiece is the audio
  source.

Bug: 23816959

Change-Id: I952c24ee51139f21a17344acd7698c9ed8f52860