Make the `reactNativeArchitectures` property more discoverable

Summary:
I've unified the function that is responsible of getting the `reactNativeArchitectures` property
to a single one (ideally we could move it inside the Gradle Plugin in the future).
I've also added a property in the `gradle.properties` file. This makes easier for users to customize the
architecture to build without having to specify a CLI flag or edit multiple gradle files.

Changelog:
[Android] [Added] - Make the `reactNativeArchitectures` property more discoverable

Reviewed By: ShikaSD

Differential Revision: D32244997

fbshipit-source-id: 33180544400f9abe63e9b539ff16fefa17a024ba