Move maps-rx snippets to app-rx snippets app

[wangela]

Currently the snippets for Maps RX are in the snippets/app (introduced in #902).

Now that we have separate snippet apps for the various libraries, we should move those files to https://github.com/googlemaps-samples/android-samples/tree/main/snippets/app-rx.

I suspect that the Rx snippets might have minSDK 24 so once these move out of snippets/app (meant for the Maps SDK for Android alone), we can revert minSDK for snippets/app to 21.