Adds an `android-dependency` option to the `update-dependency` command such that you can update Android dependencies provided the dependency and a version across relevant plugins. This PR specifically adds support for the Gradle dependency, relevant to plugin example apps.
Running the command looks like:
```
dart run script/tool/bin/flutter_plugin_tools.dart update-dependency --android-dependency gradle --version 1.2.3
```
Extracts common logic for running `pub get`, and switches commands to use it. The common logic always uses `flutter pub get` for Flutter packages, rather than `dart pub get`, since the latter will fail if someone has a non-Flutter `dart` in their path before `flutter` (e.g., Dart team members contributing PRs).
