Only set task shadow/corner for freeform tasks

Couple of changes to prevent unintended shadows on non-freeform windows:
1) Makes RelayoutParams's shadow radius take pixels instead of a
   resource to align with corner radius
2) Moves (incomplete, only checked fullscreen) windowing-mode check to
   skip shadow radius out of WindowDecoration.java into
   DesktopModeWindowDecoration.
3) Adds an INVALID_SHADOW_RADIUS to signal none should be set, also to
   align with corner radius

Fix: 375300371
Flag: EXEMPT bug fix
Test: enter split-screen, swipe up to show taskbar, verify one side
doesn't get darker (because of a shadow)

Change-Id: I9c34b3e3701faec0bb671e1b350a807fe1ee3079