Better Android Gradle Plugin 3.x integration

Summary:
Better integration with the Android Gradle-based build process, especially the changes introduced by the [Android Gradle Plugin 3.x and AAPT2](https://developer.android.com/studio/build/gradle-plugin-3-0-0-migration.html).

Fixes #16906, the `android.enableAapt2=false` workaround is no longer required.

Bases the task generation process on the actual application variants present in the project. The current manual process of iterating build types and product flavors could break down when more than one dimension type is present (see https://developer.android.com/studio/build/build-variants.html#flavor-dimensions).

This also exposes a very basic set of properties in the build tasks, so that other tasks can more reliably access them:

```groovy
android.applicationVariants.all { variant ->
    // This is the generated task itself:
    def reactBundleTask = variant.bundleJsAndAssets
    // These are the outputs by type:
    def resFileCollection = reactBundleTask.generatedResFolders
    def assetsFileCollection = reactBundleTask.generatedAssetsFolders
}
```

I've tested various combinations of product flavors and build types ([Build Variants](https://developer.android.com/studio/build/build-variants.html)) to make sure this is consistent. This is a port of what we're currently deploying to our CI process.

[ ANDROID ] [ BUGFIX ] [ react.gradle ] - Support Android Gradle Plugin 3.x and AAPT2
[ ANDROID ] [ FEATURE ] [ react.gradle ] - Expose the bundling task and its outputs via ext properties
Closes https://github.com/facebook/react-native/pull/17967

Differential Revision: D7017148

Pulled By: hramos

fbshipit-source-id: e52b3365e5807430b9caced51349abf72332a587