Hide all insets when the dream is activated

By default we hide all insets while the DreamActivity is active.
The client app has access to the main window of the activity, so it
could override this.

Bug: 151749301

Test: m && flash && start the screensaver and verify that nav bar is
gone

Change-Id: Iecdc5f67c976de5477523e9745a9e8733703c11f