Define __ANDROID_VENDOR_API__

cc modules can use __ANDROID_VENDOR_API__ to read BOARD_API_LEVEL
that is the API level of the vendor surface.

Bug: 313822931
Test: check ninja commands to have -D__ANDROID_VENDOR_API__=<version>
Change-Id: Iceb5918cbfce0f24929d02d7e3caf1e9942b88e8

android/config.go

@@ -1425,6 +1425,10 @@ func (c *config) IsVndkDeprecated() bool {
 	return !Bool(c.productVariables.KeepVndk)
 }
 
+func (c *config) VendorApiLevel() string {
+	return String(c.productVariables.VendorApiLevel)
+}
+
 func (c *deviceConfig) Arches() []Arch {
 	var arches []Arch
 	for _, target := range c.config.Targets[Android] {