Rather than normal Activities (which have a host of problems
when used for this purpose), screen savers are now a
special kind of Service that can add views to its own
special window (TYPE_DREAM, in the SCREENSAVER layer).
Dreams are now launched by the power manager; whenever it is
about to turn the screen off, it asks the window manager if
it wants to run a screen saver instead. (http://b/5677408)
Also, the new config_enableDreams bool allows the entire
feature to be switched on or off in one place. It is
currently switched off (and the APIs are all @hidden).
Change-Id: Idfe9d430568471d15f4b463cb70586a899a331f7
7d276c377c