Output bootclasspath_fragment's fragments property to sdk snapshot

Adds support for printing a PropertySet that has a property whose value
is an array of structs.

Bug: 179354495
Test: m nothing
      m conscrypt-module-sdk
      - check generated Android.bp file
Change-Id: I71be04188465610bcbea4d3c9a5e8204171a1eeb

java/bootclasspath.go

@@ -144,6 +144,8 @@ func gatherApexModulePairDepsWithTag(ctx android.BaseModuleContext, tag blueprin
 
 // ApexVariantReference specifies a particular apex variant of a module.
 type ApexVariantReference struct {
+	android.BpPrintableBase
+
 	// The name of the module apex variant, i.e. the apex containing the module variant.
 	//
 	// If this is not specified then it defaults to "platform" which will cause a dependency to be