Add symlink_outputs support to Soong.

This CL adds symlink_outputs to various locations in Soong that creates
actions that creates symlink outputs, and explicitly mark them as such.

Test: m
Bug: 160568334
Change-Id: I322751bada52a9f49011c74731d84761586e03e7
diff --git a/android/singleton.go b/android/singleton.go
index 9832378..bb6614d 100644
--- a/android/singleton.go
+++ b/android/singleton.go
@@ -168,6 +168,10 @@
 		s.buildParams = append(s.buildParams, params)
 	}
 	bparams := convertBuildParams(params)
+	err := validateBuildParams(bparams)
+	if err != nil {
+		s.Errorf("%s: build parameter validation failed: %s", s.Name(), err.Error())
+	}
 	s.SingletonContext.Build(pctx.PackageContext, bparams)
 
 }