Add defaults support for bootclasspath_fragment module type

This change transforms the bootclsspath_fragment module into a
defaultable module, in order to modify the "art-bootclasspath-fragment"
module definition based on the build flag in
https://r.android.com/2897389

Test: m nothing
Bug: 315027929
Change-Id: I01ba7b15e4fcdea9bd31c369110a3c9ee38bc9f6
diff --git a/java/java.go b/java/java.go
index 4eeded3..c5a4142 100644
--- a/java/java.go
+++ b/java/java.go
@@ -2726,6 +2726,8 @@
 		&LintProperties{},
 		&appTestHelperAppProperties{},
 		&JavaApiLibraryProperties{},
+		&bootclasspathFragmentProperties{},
+		&SourceOnlyBootclasspathProperties{},
 	)
 
 	android.InitDefaultsModule(module)