Dream settings: list items back to constant height

Expanding/collapsing items removed, settings icon shown on right
with divider if applicable.

New "Start" action on the action bar to start dreaming.  This is
no longer in preview/test mode, but a call to .dream().

Bug: 7010260
Change-Id: Ia1d8b9a70218dcc598bf62befbf545f572bbc6b1