Set up zbetagroup sourceSet

gradle tools 0.12.1 fixes merging of `android:package`

`zbetagroup` overrides the defaults for `debug` and `release`
Changing zbetagroup to productFlavor

- vanilla is the regular flavor for the google play store releases
- zbetagroup is for internal beta releases

Solves the problem of having duplicate receivers in the zbetagroup build
fix push notification issues
Merge pull request #1573 from wordpress-mobile/feature/beta-build-type

Product Flavor and AndroidManifest merge update