Improvements to empty album behavior in Gallery

Bug: 7131332
Bug: 7009464
Toasts are now immediately dismissed if a non-empty album
is selected, and dismissed once Picasa Albums load at an
otherwise empty gallery root. The alert added in a previous
prompt has been replaced with a Toast also.

Change-Id: Icca79ab264b94979eedcf78e5c3899af745d8813