Add support for product flavors

Currently, there is no support for projects with multiple product flavors. We can reproduce the issue easily by adding some dummy flavors to `build.gradle` of an arbitrary project:

	android {
		...
	    productFlavors {
	        flavorA {}
	        flavorB {}
	    }
	    ...
	}

In this case, no `testfairyXXX` tasks will be created. The plugin will fail to find the package tasks - there will be no task called `packageDebug`, instead there will be tasks called `packageFlavorADebug` and `packageFlavorBDebug`.

We can fix this issue by iterating through `applicationVariants` instead. This works even if `productFlavors` are not customized. I also modified the script to use the signing config directly from the variant.