Add a property in ndk_library for header contributions

The new property will be used to determine the header file contributions of
ndk_library(s) to the Public API surface. This should be a no-op for
regular Soong builds.

This will be used by a future bp2build converter to populate
the BUILD files for Multi-tree `cc_api_contribution` targets

(Also noticed that sdk_test.go was never added to testSrcs, which this
CL should fix)

Test: go test ./cc
Test: TH

Change-Id: Ieea093e4aac68e341c6414b6cafe02c441643cdf
diff --git a/cc/cc.go b/cc/cc.go
index f1e9bf6..99a8d7c 100644
--- a/cc/cc.go
+++ b/cc/cc.go
@@ -2350,7 +2350,10 @@
 
 		lib = GetReplaceModuleName(lib, GetSnapshot(c, &snapshotInfo, actx).HeaderLibs)
 
-		if c.IsStubs() {
+		if c.isNDKStubLibrary() {
+			// ndk_headers do not have any variations
+			actx.AddFarVariationDependencies([]blueprint.Variation{}, depTag, lib)
+		} else if c.IsStubs() {
 			actx.AddFarVariationDependencies(append(ctx.Target().Variations(), c.ImageVariation()),
 				depTag, lib)
 		} else {