Fixes MAU calculation in usage analytics in the product.

Previously we were filtering out the backend events, but as part of
was not part of the BuildCommonUsageConds in the event_logs. However
event_logs also add this condition directly in the query string, which I
did not notice.

Also, our timestamp calculation suffered from an edge case, where the
from time was not calculated right, e.g. on dates like
`2023-04-30T23:59:00Z`.

Tested locally and with customer data + added unit tests.

(cherry picked from commit ee6e66e)