Goodbye yellow brick alpha


Goodbye yellow brick alpha

Now MobileFrontend is more developed and given audience of alpha is
extremely low let's return to a stable/beta model. Existing alpha users
will continue using the beta mode as a default.

Future experiments should be done as standalone/labs experiences. They
can reach potentially even greater audiences and do not suffer from the
code debt we have in MobileFrontend so can be iterated on quickly.

So long alpha. It's been fun.

Bug: T100297
Change-Id: I3edd94bb3679e6ebd1c81476aaab292777921344