Improve notifications

[liviu-timar]

🎯 Goal

Refactor notifications to show up-to-date text and icons.

🛠 Implementation details

• Added new getOngoingCallNotification method in NotificationHandler with an onUpdate lambda parameter.
• The default implementation of getOngoingCallNotification observes the ringing state and number of members/participants and calls onUpdate with a refreshed notification that reflects the call state.
• getOngoingCallNotification is used in CallService. Will also be used in the Telecom implementation in the future.
• Various changes to DefaultNotificationHandler and CallService
• Changed notification titles, desc, icons.
• ❗ Breaking change: the callDisplayName parameter in NotificationHandler#getRingingCallNotification() is now nullable (String -> String?).

🎨 UI Changes

🧪 Testing

• Go through the entire ringing flow and watch all notifications: incoming, outgoing, ongoing.
• Start a group call and add/remove participants: incoming, outgoing, ongoing.

[liviu-timar]

Found a bug. Pressing on the ongoing call Hang Up button does not work on the caller device, only on receiver. Fixing it.