Make AmpMatchingStrategyArg discriminents start at 1

[bendk]

This is the easiest way to land https://bugzilla.mozilla.org/show_bug.cgi?id=1924764.

The Desktop code for this currently assumes that discrimenents start at one. That's how it's worked historically, but that behavior was never correct. This allows us to fix the bug but not have to update the suggest desktop code.

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Branch builds: add [firefox-android: branch-name] to the PR title.

[0c0w3]

Thanks!