Let tor-browser-build specify the correct channel

Currently, we have the following lines in browser/config/mozconfigs/base-browser-android:

    # We only use beta GeckoView for now, for official builds
    ac_add_options --enable-update-channel=beta

This isn't correct.

We actually build as beta, and then we override Moz files with our aars.

Instead, we should define the proper channel on tor-browser-build, publish our GV locally, and pass it to firefox-android with the mavenLocal repository (like the local publication for development purposes: https://github.com/mozilla-mobile/firefox-android/blob/main/fenix/docs/substituting-local-gv.md).