* chore: Adding samples for V3.
Adding samples for V3 BETA in a new module `app-v3`. These samples are
generated by the `generateV3` Gradle task which runs when the `app`
module is built.
TODOs left:
- [ ] Update README with a note about V3 samples
* Update README and location of secure.properties.
* Using product flavors instead of creating a new module for V3 & GMS.
* Update build workflow to run lint and assemble on all flavor/build types.
* Not using deprecated API.
* Install Android NDK.
* Update license headers.
