Use system density for window decoration

There is a prototype feature to allow overriding density for desktop
tasks. This override should not apply to the window decoration.
If the override system property is detected, use the system density for
the window decoration.

Bug: 272529050
Test: set 272 dpi for desktop tasks, move a fullscreen task to desktop,
  observe that task is using a different density, but window decoration
  is using the system density
Test: remove desktop task density override, observe that desktop task
  window decoration using task density (which matches system)
Change-Id: I1c8db490e527381b2adaa0a657050822763e1516