Update project structure

This was done in comparison to a newly created flutter project. Notably
this included migrating to AndroidX, updating Gradle and Kotlin and
introducing a new .gitignore file in the android subdirectory.

.gitignore

@@ -24,50 +24,8 @@
 .dart_tool/
 pubspec.lock
 .flutter-plugins
+.flutter-plugins-dependencies
 .packages
 .pub-cache/
 .pub/
 /build/
-
-# Android related
-**/android/.gradle
-**/android/gradle
-**/android/captures/
-**/android/gradlew
-**/android/gradlew.bat
-**/android/local.properties
-**/android/**/GeneratedPluginRegistrant.java
-
-# iOS/XCode related
-**/ios/**/*.mode1v3
-**/ios/**/*.mode2v3
-**/ios/**/*.moved-aside
-**/ios/**/*.pbxuser
-**/ios/**/*.perspectivev3
-**/ios/**/*sync/
-**/ios/**/.sconsign.dblite
-**/ios/**/.tags*
-**/ios/**/.vagrant/
-**/ios/**/DerivedData/
-**/ios/**/Icon?
-**/ios/**/Pods/
-**/ios/**/.symlinks/
-**/ios/**/profile
-**/ios/**/xcuserdata
-**/ios/.generated/
-**/ios/Flutter/App.framework
-**/ios/Flutter/Flutter.framework
-**/ios/Flutter/Generated.xcconfig
-**/ios/Flutter/app.flx
-**/ios/Flutter/app.zip
-**/ios/Flutter/flutter_assets/
-**/ios/Flutter/flutter_export_environment.sh
-**/ios/ServiceDefinitions.json
-**/ios/Runner/GeneratedPluginRegistrant.*
-
-# Exceptions to above rules.
-!**/ios/**/default.mode1v3
-!**/ios/**/default.mode2v3
-!**/ios/**/default.pbxuser
-!**/ios/**/default.perspectivev3
-!/packages/flutter_tools/test/data/dart_dependencies_test/**/.packages