Fix WindowInsets#getStableInsets

WindowInsets#getStableInsets should return system bar insets as if
system bars are visible, regardless of the window flags.

Fix: 228807465
Test: See if a window with FLAG_LAYOUT_NO_LIMITS or FLAG_FULLSCREEN can
      receive all the system bar insets.
Change-Id: Ib8fff5ba78c8f52141a048027821ef182d2f7992