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
diff --git a/snapshot/Android.bp b/snapshot/Android.bp
index c7e9d7e..f17ac53 100644
--- a/snapshot/Android.bp
+++ b/snapshot/Android.bp
@@ -15,6 +15,7 @@
         "recovery_snapshot.go",
         "snapshot.go",
         "snapshot_base.go",
+        "util.go",
         "vendor_snapshot.go",
     ],
     pluginFor: ["soong_build"],