Hide EME site permission

In 9d5afd50, Mozilla exposed support for EME. It is only shown as available on certain Android devices where there is hardware support, and on those devices it is gated by a site permission.

I looked at this a bit, and it seems tor-browser#16285 is still protecting us here.

As a test, I used https://cpearce.github.io/mse-eme/

We should hide this new site permission, so it doesn't confuse people.