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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