Element Call ringing notifications (#2978)

- Add `ActiveCallManager` to handle incoming and ongoing calls.
- Add ringing call notifications with full screen intents and missed call ones as part of the 'conversation' notifications.

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Co-authored-by: ElementBot <benoitm+elementbot@element.io>

libraries/designsystem/src/main/kotlin/io/element/android/libraries/designsystem/components/avatar/AvatarSize.kt

@@ -22,6 +22,7 @@ import androidx.compose.ui.unit.dp
 enum class AvatarSize(val dp: Dp) {
     CurrentUserTopBar(32.dp),
 
+    IncomingCall(140.dp),
     RoomHeader(96.dp),
     RoomListItem(52.dp),