Update Headers_only to a bool pointer

This makes it easier to identify intentionally setting the property

Test: m nothing
Change-Id: Id89d1e64da3809025f63cafe7557e57a80e0ffee
diff --git a/java/base.go b/java/base.go
index ec98dfe..03198b5 100644
--- a/java/base.go
+++ b/java/base.go
@@ -194,7 +194,7 @@
 	Generated_srcjars []android.Path `android:"mutated"`
 
 	// If true, then only the headers are built and not the implementation jar.
-	Headers_only bool
+	Headers_only *bool
 }
 
 // Properties that are specific to device modules. Host module factories should not add these when
@@ -582,7 +582,7 @@
 
 func (j *Module) checkHeadersOnly(ctx android.ModuleContext) {
 	if _, ok := ctx.Module().(android.SdkContext); ok {
-		headersOnly := proptools.Bool(&j.properties.Headers_only)
+		headersOnly := proptools.Bool(j.properties.Headers_only)
 		installable := proptools.Bool(j.properties.Installable)
 
 		if headersOnly && installable {
@@ -1180,7 +1180,7 @@
 	flags.classpath = append(android.CopyOf(extraClasspathJars), flags.classpath...)
 
 	// If compiling headers then compile them and skip the rest
-	if j.properties.Headers_only {
+	if proptools.Bool(j.properties.Headers_only) {
 		if srcFiles.HasExt(".kt") {
 			ctx.ModuleErrorf("Compiling headers_only with .kt not supported")
 		}