Upgrade dependencies, android sdk, flutter, gradle, etc

flutter/TODO.md

@@ -27,6 +27,13 @@
 
  - [ ] fix time format (in message-list, in card, top right) - midnight is shown as "24:05" instead of "00:05" - thats weird
 
+ - [ ] Add scrollbar
+       -> https://api.flutter.dev/flutter/material/Scrollbar-class.html
+
+ - [ ] you cant unsubscribe from foreign channel without completely loosing subscription.
+       perhaps subscriptions should have two cofirmed bool (both must be true to receive messages): confirmed-owner && confirmed-subscriber
+       Then the subscriber can unconfirm his half - without loosing the owner confirmation
+
  -----
 
 # TODO iOS specific