Implement DoubleShadowIconDrawable for use in dream complications

Also refactors the code to place the double shadow code in a shared
location, and specify the shadow config in xml instead of hardcoding.

Bug: 239089588
Test: manually on device by flashing and verifying the icon atop the
dream has the proper shadow

Change-Id: I18db07c243e20785409d5cc760b614a8ee775f1f