Delay global alerts by 1s for high thresholds

By skipping updates if an alert is already scheduled, this avoids firing
the global alert too often on devices with high transfer speeds and high
quota.

Test: with tethering watching videos. Also runtest frameworks-net
Bug: 117243748
Change-Id: Idce6059832db7a0e4a7117bbb3e424ec62ae3b21