* Teach flutter tools to find gradle
Flutter tools will now use Gradle from Android Studio, which is now found automatically.
flutter doctor will verify that Android Studio has been installed, and that the included Gradle is at least version 2.14.1.
It is still possible to manually configure the path to Android Studio (flutter config --android-studio-dir=XXX) or Gradle (flutter config --gradle-dir=XXX), but this should only be necessary if they're installed somewhere non-standard.
Only tested on Linux and macOS for now.
Fixes#8131
With this change, they're run via instance methods on an object
obtained through the context. This will allow us to substitute
that object in tests with replay/record versions to allow us to
mock out the os-layer in tests.
