Update DreamOverlayService to work with Headless System User Mode.

The DreamOverlayService must run as part of the system user SystemUI
instance as the user-specific instance only has secondary components
enabled. As a result, the added overlay window must be set to show
for all users.

Test: atest DreamOverlayServiceTest#testSystemFlagShowForAllUsersSetOnWindow
Test: manual - verified overlay appears when dream shown in HSUM
Fixes: 283486171
Change-Id: I14d3335cb0d2a78780258300737f5d0dbe8d97e1