Update prebuilt_etc available for snapshot

Make prebuilt_etc module available for the snapshot. This change
includes implementing snapshot interface for the prebuilt_etc module so
it can be added to the snapshot from the fake snapshot, or when the
module is specified in the list.

Bug: 192430376
Test: m nothing passed
Test: Checked if the module is included in the snapshot properly
Change-Id: I3574d2a1b8f8f4e5f083f3913e8768f5088d0c46

snapshot/Android.bp

@@ -15,6 +15,7 @@ bootstrap_go_package {
         "recovery_snapshot.go",
         "snapshot.go",
         "snapshot_base.go",
+        "util.go",
         "vendor_snapshot.go",
     ],
     pluginFor: ["soong_build"],

snapshot/util.go

@@ -0,0 +1,36 @@
+// Copyright 2021 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+package snapshot
+
+import "android/soong/android"
+
+func WriteStringToFileRule(ctx android.SingletonContext, content, out string) android.OutputPath {
+	outPath := android.PathForOutput(ctx, out)
+	android.WriteFileRule(ctx, outPath, content)
+	return outPath
+}
+
+func CopyFileRule(pctx android.PackageContext, ctx android.SingletonContext, path android.Path, out string) android.OutputPath {
+	outPath := android.PathForOutput(ctx, out)
+	ctx.Build(pctx, android.BuildParams{
+		Rule:        android.Cp,
+		Input:       path,
+		Output:      outPath,
+		Description: "copy " + path.String() + " -> " + out,
+		Args: map[string]string{
+			"cpFlags": "-f -L",
+		},
+	})
+	return outPath
+}