Revert "Update app icon shadows"

Revert submission 17935777-lightDarkDetection

Reason for revert: Talked with Adam Cohen, white shadows are controverstial and we'd need to escalate to senior UX if we want to ship. James would rather do it in U timeframe, will flag to UXers we work with that the opportunity to escalate is available if needed.
Reverted Changes:
Ia91221427:Update light/dark detection.
Ie12d3d054:Update app icon shadows
Icc88c3496:Update smartspace shadow

Change-Id: I8bc90e79073b8e088f2912fef2af4c6c99a33553