[w3c/gamepad] Make permissions policy allow list "all" (Issue #155)

Given the breakage that was found when Gecko switched to `self` (and Gecko changing to `all`), we should update the spec to use `all` instead. 

Even though the API remains enabled for "all", it at least makes this a policy-controlled feature (allowed those that want to disable it to do so). 

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https://github.com/w3c/gamepad/issues/155