UI Tweaks to Zen Mode Voice Activity.

Small cleanup of the UI / strings.
- Cleanup of string constant names.
- Add a specific string for ZenModeVoiceActivity label.
- Change look of list position indicators.
- Add optional header to VoiceSettingsActivity.
- Fix NPE in some cases when disabling Zen mode.

Change-Id: Ic09ee2b1b5a50891b5447c2db0e3de3c475696bf