Update the documentation for onResume of Activity

onResume is not always an indicator that the activity is visible and
start interacting with the user. The case demonstrated activity is in
the relaunched process (onCreate() -> onResume()) while the screen is
still locked. Update the documentation to prevent developers from
being confused.

Bug: 187494420
Test: build
Change-Id: Id52a02b54218d899a948640049cb627eb98ef2f6