Export SOONG_CC_API_XML to Make outside androidmk

This relands Ie0a945d879de4f99ce76d005aea8041719c244f7 with a fix
to prevent building XMl files for multiple variants of a library.

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: forrest
Change-Id: I54710c00901976a736e88126f406e02b1f3c3586