Set flag exporter provider for vendor snapshots

Also, TestVendorSnapshotUse is amended so it can test whether vndk
snapshot and vendor snapshot modules export their flags correctly.

Bug: N/A
Test: manual test
Change-Id: Ie62ad1206449b1a40db8a2282af00616db693017
diff --git a/cc/vendor_snapshot.go b/cc/vendor_snapshot.go
index 4c206e6..64e3ea8 100644
--- a/cc/vendor_snapshot.go
+++ b/cc/vendor_snapshot.go
@@ -241,6 +241,8 @@
 		})
 	}
 
+	p.libraryDecorator.flagExporter.setProvider(ctx)
+
 	return in
 }