Mobile performance improvements (#417)

* First performance tweaks (caching and rendering improvemetns)

* Revert asset response caching

* 3-step image loading in asset viewer

* Prevent panning and zooming until full-scale version is loaded

* Loading indicator

* Adapt to gallery PR

* Cleanup

* Dart format

* Fix exif sheet

* Disable three stage loading until settings are available

mobile/pubspec.lock

@@ -328,6 +328,13 @@ packages:
       url: "https://pub.dartlang.org"
     source: hosted
     version: "3.3.0"
+  flutter_displaymode:
+    dependency: "direct main"
+    description:
+      name: flutter_displaymode
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "0.4.0"
   flutter_hooks:
     dependency: "direct main"
     description: