Add logtags source file support

Add support for converting logtags files to source files.  Each
.logtags file is converted to a .java file, but only after all
.logtags files have been combined into a merged text file by
the logtagsSingleton.

Change-Id: I375d82874029bd26a7c528e46add2a638ba123f9
diff --git a/cmd/soong_build/main.go b/cmd/soong_build/main.go
index a1a4ce0..83205ba 100644
--- a/cmd/soong_build/main.go
+++ b/cmd/soong_build/main.go
@@ -76,6 +76,7 @@
 	// Singletons
 	ctx.RegisterSingletonType("checkbuild", common.CheckbuildSingleton)
 	ctx.RegisterSingletonType("env", common.EnvSingleton)
+	ctx.RegisterSingletonType("logtags", java.LogtagsSingleton)
 
 	configuration, err := common.NewConfig(srcDir)
 	if err != nil {