Integrate Element Call with widget API (#1581)

* Integrate Element Call with widget API.

- Add `appconfig` module and extract constants that can be overridden in forks there.
- Add an Element Call feature flag, disabled by default.
- Refactor the whole `ElementCallActivity`, move most logic out of it.
- Integrate with the Rust Widget Driver API (note the Rust SDK version used in this PR lacks some needed changes to make the calls actually work).
- Handle calls differently based on `CallType`.
- Add UI to create/join a call.

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

libraries/featureflag/api/src/main/kotlin/io/element/android/libraries/featureflag/api/FeatureFlags.kt

@@ -55,4 +55,10 @@ enum class FeatureFlags(
         description = "Allow user to lock/unlock the app with a pin code or biometrics",
         defaultValue = false,
     ),
+    InRoomCalls(
+        key = "feature.elementcall",
+        title = "Element call in rooms",
+        description = "Allow user to start or join a call in a room",
+        defaultValue = false,
+    )
 }