exports LOCAL_SOONG_VNDK_VERSION for VNDK libs

For vndk_prebuilt_shared module, it is set by 'version' property.
For other vndk libs(e.g. cc_library with vndk.enabled), it is set as
PLATFORM_VNDK_VERSION.

Background:
To support "skip installing current VNDK".

You can get a system.img without current VNDK libs.
This may help when you want a smaller system.img given that the image
will be used with a specific version (not current) of vendor image.

Bug: 132140714
Test: m TARGET_SKIP_CURRENT_VNDK=true && see if current VNDK is not
installed

Change-Id: I1c603efc3e95fe8bdf870f2de91994077899fca7
diff --git a/cc/cc.go b/cc/cc.go
index a3b9a92..96acf81 100644
--- a/cc/cc.go
+++ b/cc/cc.go
@@ -534,6 +534,13 @@
 	return false
 }
 
+func (c *Module) vndkVersion() string {
+	if vndkdep := c.vndkdep; vndkdep != nil {
+		return vndkdep.Properties.Vndk.Version
+	}
+	return ""
+}
+
 func (c *Module) isPgoCompile() bool {
 	if pgo := c.pgo; pgo != nil {
 		return pgo.Properties.PgoCompile