Bug 921056 - Visual polish for status bar

- CallScreen.isStatusBarMode to be DRY-er
- HandledCall.formatPhoneNumber will format according to isStatusBarMode
- Don't display lockscreen elements and group call details in status bar
- Use new icons
- Adjust spacing
- Held mode should be dark gray
Merge pull request #12658 from Rik/polish-statusbar-921056

Bug 921056 - Visual polish for status bar r=etienne