report notification load on panel reveal

Counts may differ from user perception. For example, if notifications arrive
after the shade is open (even if it is only peeking) there will not be another
panel_reveal before the user sees the shade.  User perception is more accurately
measured by visibility events.

Peek events will report the notificaiton load as 1.

Bug: 20088581
Change-Id: I10221d4b66a18c223aae21e616615f087c65b1e1