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>

features/logout/api/src/main/res/values/localazy.xml

@@ -1,8 +1,12 @@
 <?xml version="1.0" encoding="utf-8"?>
 <resources xmlns:xliff="urn:oasis:names:tc:xliff:document:1.2">
+  <string name="screen_signout_backing_up_subtitle">"Please wait for this to complete before signing out."</string>
+  <string name="screen_signout_backing_up_title">"Your keys are still being backed up"</string>
   <string name="screen_signout_confirmation_dialog_content">"Are you sure you want to sign out?"</string>
   <string name="screen_signout_confirmation_dialog_title">"Sign out"</string>
   <string name="screen_signout_in_progress_dialog_content">"Signing out…"</string>
+  <string name="screen_signout_last_session_subtitle">"You are about to sign out of your last session. If you sign out now, you might lose access to your encrypted messages."</string>
+  <string name="screen_signout_last_session_title">"Have you saved your recovery key?"</string>
   <string name="screen_signout_confirmation_dialog_submit">"Sign out"</string>
   <string name="screen_signout_preference_item">"Sign out"</string>
 </resources>