Remove usage of low profile in DreamService

The low profile in DreamService was used to hide the status bar and
navigation bar. Now this is done by
WindowInsetController.hide(systemBars()) in DreamActivity.

Bug: 151749301

Test: m && flash && verify that system bars don't show up
Change-Id: I8fc9ca67262e92422afb472ccd76ec561c90f66d