Postpone reporting fully drawn until app is drawn

"Fully drawn" should stand for an usable state of the application.
If its window hasn't drawn, the early reported time may not close
to the real use case.

This change ensures that "Fully drawn" is at least >= "Displayed".

Bug: 131713448
Test: atest ActivityMetricsLaunchObserverTests#testOnReportFullyDrawn
Test: Launch an application (e.g. Settings) that invokes
      Activity#reportFullyDrawn in an early stage. The printed
      duration of "Displayed" and "Fully drawn" should show
      the same time.

Change-Id: Id37425436dc564fc657f0777db8e0f056bbb748d