Put Smartspace on overlay behind a flag.

This change checks a flag before registering Smartspace on the dream
overlay.

Flag: HIDE_SMARTSPACE_ON_DREAM_OVERLAY

Bug: 270169281
Bug: 270223352
Test: atest SmartSpaceComplicationTest
Test: flipped on the flag and no longer see smartspace on dream

Change-Id: Iad8482b9e683a7bc9efdee70c1b559b1937f487f