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/androidmk.go b/java/androidmk.go
index b7e2d2f..97b303d 100644
--- a/java/androidmk.go
+++ b/java/androidmk.go
@@ -20,6 +20,8 @@
 	"strings"
 
 	"android/soong/android"
+
+	"github.com/google/blueprint/proptools"
 )
 
 func (library *Library) AndroidMkEntriesHostDex() android.AndroidMkEntries {
@@ -79,7 +81,7 @@
 	} else if !library.ApexModuleBase.AvailableFor(android.AvailableToPlatform) {
 		// Platform variant.  If not available for the platform, we don't need Make module.
 		entriesList = append(entriesList, android.AndroidMkEntries{Disabled: true})
-	} else if library.properties.Headers_only {
+	} else if proptools.Bool(library.properties.Headers_only) {
 		// If generating headers only then don't expose to Make.
 		entriesList = append(entriesList, android.AndroidMkEntries{Disabled: true})
 	} else {