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/cc/vendor_snapshot.go b/cc/vendor_snapshot.go
index 6705d55..ba4d79f 100644
--- a/cc/vendor_snapshot.go
+++ b/cc/vendor_snapshot.go
@@ -210,9 +210,9 @@
 		if fake {
 			// All prebuilt binaries and headers are installed by copyFile function. This makes a fake
 			// snapshot just touch prebuilts and headers, rather than installing real files.
-			return writeStringToFileRule(ctx, "", out)
+			return snapshot.WriteStringToFileRule(ctx, "", out)
 		} else {
-			return copyFileRule(ctx, path, out)
+			return snapshot.CopyFileRule(pctx, ctx, path, out)
 		}
 	}
 
@@ -350,7 +350,7 @@
 			ctx.Errorf("json marshal to %q failed: %#v", propOut, err)
 			return nil
 		}
-		ret = append(ret, writeStringToFileRule(ctx, string(j), propOut))
+		ret = append(ret, snapshot.WriteStringToFileRule(ctx, string(j), propOut))
 
 		return ret
 	}