Add Dimmer to RootDisplayArea

There are 2 reasons that led to this change.

Right now on trusted displays Dimmer is offered on windowed
magnification display area, but no one offers it on untrusted displays.
There is no guarantee that windowed manification display area always
exists either on all OEM.

In Task#getDimmer() if the task is fullscreen but is translucent it asks
for a dimmer from one of its ancestors, and WindowState#applyDims() uses
Dimmer as if it always can get one. There can also be system dialogs
(e.g. ANR dialogs) that don't belong to any TaskDisplayArea that also
needs a Dimmer.

Therefore add a dimmer to RootDisplayArea.

Bug: 186436456
Test: atest TaskTests
Change-Id: I284b60124e27496352c27f446a2742e3e4756ec9