Only stop observing device rotation in `stop`
Only stop observing device rotation in `stop`

Fixes bug (possibly related to #5) that occurred if:
- VolumeBar started
- App backgrounded (causing `VolumeBar` to call `stop`)
- App becomes active

Since we were removing all notifications in the `stop` method, `VolumeBar` wouldn’t receive the `applicationDidBecomeActive` notification when the app re-entered the foreground, so `VolumeBar` wouldn’t be restarted.

gizmosachin/VolumeBar,gizmosachin/VolumeBar,gizmosachin/VolumeBar