Introduce security checks to startDreamActivity

This CL checks that the caller is the dream package and that the
device is dreaming when the activity is started.

isDream was instroduced in order to exempt the DreamActivity from
background start check. setAllowBackgroundActivityStart does that
more cleanly.

Bug: 152281628

Test: m && flashall && verify dream works

Change-Id: Id6161f923f3350d482b3452a78a792a541dcf1a1