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

etc/Android.bp

@@ -9,6 +9,7 @@ bootstrap_go_package {
         "blueprint",
         "soong",
         "soong-android",
+        "soong-snapshot",
     ],
     srcs: [
         "prebuilt_etc.go",