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.

---------

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

libraries/preferences/test/src/main/kotlin/io/element/android/libraries/featureflag/test/InMemoryPreferencesStore.kt

@@ -23,9 +23,11 @@ import kotlinx.coroutines.flow.MutableStateFlow
 class InMemoryPreferencesStore(
     isRichTextEditorEnabled: Boolean = false,
     isDeveloperModeEnabled: Boolean = false,
+    customElementCallBaseUrl: String? = null,
 ) : PreferencesStore {
     private var _isRichTextEditorEnabled = MutableStateFlow(isRichTextEditorEnabled)
     private var _isDeveloperModeEnabled = MutableStateFlow(isDeveloperModeEnabled)
+    private var _customElementCallBaseUrl = MutableStateFlow(customElementCallBaseUrl)
 
     override suspend fun setRichTextEditorEnabled(enabled: Boolean) {
         _isRichTextEditorEnabled.value = enabled
@@ -43,6 +45,14 @@ class InMemoryPreferencesStore(
         return _isDeveloperModeEnabled
     }
 
+    override suspend fun setCustomElementCallBaseUrl(string: String?) {
+        _customElementCallBaseUrl.tryEmit(string)
+    }
+
+    override fun getCustomElementCallBaseUrlFlow(): Flow<String?> {
+        return _customElementCallBaseUrl
+    }
+
     override suspend fun reset() {
         // No op
     }