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>

settings.gradle.kts

@@ -52,6 +52,7 @@ enableFeaturePreview("TYPESAFE_PROJECT_ACCESSORS")
 rootProject.name = "ElementX"
 include(":app")
 include(":appnav")
+include(":appconfig")
 include(":tests:konsist")
 include(":tests:uitests")
 include(":tests:testutils")