Export SOONG_CC_API_XML to Make outside androidmk

The SOONG_CC_API_XML values are needed for modules that are not
exported to Make, export them from a singleton that covers all
modules instead of an AndroidMkProvider that may not be called
for some modules.

Bug: 193819970
Test: manual
Change-Id: Ie0a945d879de4f99ce76d005aea8041719c244f7