Fix ScanManagerTest flakiness

Fix flakiness for testMetricsScanRadioWeightedDuration by adding more
margin since rare failure was due to longer scan duration depending on
system load. And fix testMetricsScanRadioDurationScreenOnOff by applying
a looser check since some extra scan on/off may occure when higher duty
cycle scan is turned on/off

Bug: 330461915
Bug: 326336517
Test: atest ScanManagerTest
Flag: EXEMPT, test change only
Change-Id: I07ec1309b6312f446eb137bc0ec68b801f4a1342