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/variable.go

@@ -508,6 +508,8 @@ type ProductVariables struct {
 	ArtTargetIncludeDebugBuild *bool `json:",omitempty"`
 
 	SystemPropFiles []string `json:",omitempty"`
+
+	EnableUffdGc *string `json:",omitempty"`
 }
 
 type PartitionQualifiedVariablesType struct {