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/impl/src/main/kotlin/io/element/android/libraries/featureflag/impl/StaticFeatureFlagProvider.kt

@@ -37,6 +37,7 @@ class StaticFeatureFlagProvider @Inject constructor() :
                 FeatureFlags.NotificationSettings -> true
                 FeatureFlags.VoiceMessages -> false
                 FeatureFlags.PinUnlock -> false
+                FeatureFlags.InRoomCalls -> false
             }
         } else {
             false