Review Mozilla 1978008: Refactor MetricsUtils to remove direct dependency from AdvertisingIdClient and Base64 encoding implementations.

🔍 Bugzilla Audit

Bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1978008

Conflicted in the 143 rebase, and it added more uses of AdvertisingIdClient, which would be problematic at compile time.

Bea fixed them, but there was a refactor we might want to check a little bit.

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