Export cc vendor functions for usage by rust.

This CL exports and refactors some cc vendor-snapshot related functions
so they can be reused by rust modules to support vendor snapshotting.

Bug: 184042776
Test: m nothing
Change-Id: I12706e62ce0ac3b2b4298085fafc1d77b8e0a0c4
diff --git a/cc/genrule.go b/cc/genrule.go
index 82d7205..b0efc6c 100644
--- a/cc/genrule.go
+++ b/cc/genrule.go
@@ -103,7 +103,7 @@
 		// If not, we assume modules under proprietary paths are compatible for
 		// BOARD_VNDK_VERSION. The other modules are regarded as AOSP, that is
 		// PLATFORM_VNDK_VERSION.
-		if vndkVersion == "current" || !isVendorProprietaryModule(ctx) {
+		if vndkVersion == "current" || !IsVendorProprietaryModule(ctx) {
 			variants = append(variants, VendorVariationPrefix+ctx.DeviceConfig().PlatformVndkVersion())
 		} else {
 			variants = append(variants, VendorVariationPrefix+vndkVersion)