Set flag exporter provider for vndk_prebuilt libs

vndk_prebuilt exports some flags, so the provider should be set.

Bug: N/A
Test: m
Change-Id: I6c4df4139e66f992b63686adfe0635f08bafa649

cc/vndk_prebuilt.go

@@ -169,6 +169,8 @@ func (p *vndkPrebuiltLibraryDecorator) link(ctx ModuleContext,
 			TableOfContents: p.tocFile,
 		})
 
+		p.libraryDecorator.flagExporter.setProvider(ctx)
+
 		return in
 	}