Replace android.WriteFile rule with android.WriteFileRule

The android.WriteFile rule takes careful escaping to produce the
right contents.  Wrap it in an android.WriteFileRule that handles
the escaping.

Test: compare all android.WriteFile outputs
Change-Id: If71a5843af47a37ca61714e1a1ebb32d08536c31
diff --git a/cc/snapshot_utils.go b/cc/snapshot_utils.go
index 238508d..05c06ac 100644
--- a/cc/snapshot_utils.go
+++ b/cc/snapshot_utils.go
@@ -93,13 +93,6 @@
 
 func writeStringToFile(ctx android.SingletonContext, content, out string) android.OutputPath {
 	outPath := android.PathForOutput(ctx, out)
-	ctx.Build(pctx, android.BuildParams{
-		Rule:        android.WriteFile,
-		Output:      outPath,
-		Description: "WriteFile " + out,
-		Args: map[string]string{
-			"content": content,
-		},
-	})
+	android.WriteFileRule(ctx, outPath, content)
 	return outPath
 }