8 bugs related to voip calling in Element v1.0

Version: Element 1.0, F-Droid version
Test Platform: Two Huawei EMUI 10 (Android 10) Phone, One computer running Elements Desktop 1.7.1

- Bug1: When receiving calls, customized ringtone is played with default sound ring.ogg (on the phone).
- Bug2: incoming-call window won't pop up. and the only thing shown is a notification (on the phone).
- Bug3: If I open the app during an incoming call, there's only a hangup button inside that room (no incoming-call window popup, and no accept button inside room) (on the phone)
- Bug4: Login the same account in phone1 and computer, and use phone2 to call phone1. Then, reject the call on the computer. Incoming-call notification still persists on phone1. If you click on that notification and open incoming-call window, accept/hangup button persists (ringtone stops though). At this time if you click accept/reject button, those two buttons will disappear and the incoming-call window won't disappear.
- Bug5: While calling on the phone, it plays default call ringtone (ring.ogg) on caller's phone rather than normal call request sound.
- Bug6: If I exit element on android during a video call, there won't be a floating window showing peer video footage even if drop-zone permission is granted. (The app won't notify dropzone permission is enabled either)
- Bug7: Customized ringtone only play once and silence after that. (Here silence means customized ringtone stops playing and according to bug1, ring.ogg still plays)
- Bug8: See [rajil's comment](https://github.com/vector-im/riotX-android/issues/1719#issuecomment-660293291)