Refactor the Timer class to use a custom key type instead of number

[rsimha]

This came out of the discussion at #21618 (comment)

We currently use number as the default type in the Timer class. We should use a custom key type instead, that will make typecasts elsewhere in the code either unnecessary, or less ambiguous.

amphtml/src/service/timer-impl.js

Line 67 in a352b31

* @return {number|string}

amphtml/src/service/timer-impl.js

Line 96 in a352b31

* @param {number|string|null} timeoutId

[rsimha]

/cc @jridgewell @aghassemi

[dreamofabear]

I assume this was @jridgewell's suggestion.

[stale]

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