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

android/singleton.go

@@ -168,6 +168,10 @@ func (s *singletonContextAdaptor) Build(pctx PackageContext, params BuildParams)
 		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)
 
 }