Conditionally pass kernel version to build.prop

This is to replicate Makefile's behavior as-is.

Bug: 322090587
Test: build and compare build command
Change-Id: Ib9ebec920701ea561a007bf2a1ae092ae1b4ff28

android/config.go

@@ -2081,3 +2081,7 @@ func (c *config) UseDebugArt() bool {
 func (c *config) SystemPropFiles(ctx PathContext) Paths {
 	return PathsForSource(ctx, c.productVariables.SystemPropFiles)
 }
+
+func (c *config) EnableUffdGc() string {
+	return String(c.productVariables.EnableUffdGc)
+}