Populate source-event-delay time to AppStartupHelper

The metrics can provide source_event_delay_millis_{$packageName}
if the caller sets the start source information.

Bug: 166614700
Test: CollectorsHelperTest: \
      AppStartupHelperTest#testSingleColdLaunchMetric
Change-Id: If2517b5ae8c8a38c1bc5c6ea8dc4e249b3201d29