Setup a `newArchEnabled` property to Opt-in the New Architecture in the template (#32790)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/32790

As the name says, I've added a `newArchEnabled` property that can be used to toggle the
New Architecture in the new app template. Users can use this to try the New Architecture in their
project by either:
* Set `newArchEnabled` to true inside the `gradle.properties` file
* Invoke gradle with `-PnewArchEnabled=true`
* Set an environment variable `ORG_GRADLE_PROJECT_newArchEnabled=true`

The Project property will also control if:
* ReactNative dependency should be built from source or not.
* The NDK should be enabled or not.

Please note that this requires RN 0.68.x to run properly (it won't work with 0.67 RCs) or a Nightly version of React Native.

Changelog:
[Android] [Added] - Setup a `newArchEnabled` property to Opt-in the New Architecture in the template

Reviewed By: ShikaSD

Differential Revision: D33065373

fbshipit-source-id: 32085f5b071d6243936bafd91425b5b43e5b5101