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/preferences/api/src/main/kotlin/io/element/android/features/preferences/api/store/PreferencesStore.kt

@@ -25,5 +25,8 @@ interface PreferencesStore {
     suspend fun setDeveloperModeEnabled(enabled: Boolean)
     fun isDeveloperModeEnabledFlow(): Flow<Boolean>
 
+    suspend fun setCustomElementCallBaseUrl(string: String?)
+    fun getCustomElementCallBaseUrlFlow(): Flow<String?>
+
     suspend fun reset()
 }