This migrator will upgrade the minSdkVersion used in the [module-level build.gradle](https://developer.android.com/build#module-level) file to flutter.minSdkVersion. 

The PR also makes a small refactor to `AndroidProject` to add a getter for the module level build.gradle file, and uses that getter in places where we were getting that file (previously it was being gotten directly via `hostAppGradleRoot.childDirectory('app').childFile('build.gradle')`.

Part of the work for deprecating support for the Jelly Bean android apis.