1) Assuming I have an untrusted app, e.g. sideloaded from an untrusted source with permissions to access storage granted installed on a recent (v7 or 8.x) Android device.
2) Assume I also have Signal, Wire or Telegram etc. installed from the Google Play app store on the same device.
Could the untrusted app access either the stored messages, or the screen contents and keypresses when in the secure messaging app? Or is there some other method of compromising the supposedly secure messages?