Avoid useless time fetch in DataUpdateCoordinator

[bdraco]

Proposed change

Since we used the async_call_at helper, it would always call dt_util.utcnow() to feed the _handle_refresh_interval which threw it away. This meant we had to fetch time twice as much as needed for each update.

We spent a lot of effort in the past to reduce the amount of time calls because there are ARM platforms with errata which can't use vdso. On these platforms time calls are about an order of magnitude slower because time calls are full syscalls (which includes the Blue)

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