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/defs.go

@@ -69,8 +69,9 @@ var (
 	// A symlink rule.
 	Symlink = pctx.AndroidStaticRule("Symlink",
 		blueprint.RuleParams{
-			Command:     "rm -f $out && ln -f -s $fromPath $out",
-			Description: "symlink $out",
+			Command:        "rm -f $out && ln -f -s $fromPath $out",
+			Description:    "symlink $out",
+			SymlinkOutputs: []string{"$out"},
 		},
 		"fromPath")