Ensure that dreams are "supported" in service tests

When this test is run on a device that has dreams disabled in its configurations, the test will fail even though the service's ability to pass the test is independent from the device's capabilities. Faking these settings to enable dreams allows the test to always run properly.

Bug: 267812189
Change-Id: Id8db1e3f0107c6544303c596189878b27a216594
Test: ALLOW_MISSING_DEPENDENCIES=true atest FrameworksServicesTests:com.android.server.power.PowerManagerServiceTest#testScreensaverActivateOnSleepEnabled_powered_afterTimeout_goesToDreaming