AAPT2: propagate SPEC_OVERLAYABLE to splits

A resource marked SPEC_OVERLAYABLE in the base package was not marked as
such in the corresponding split packages. Add missing copy statement.

Test: manual (aapt2 dump test content in libandroidfw)
Change-Id: I800e92111a9da7c6f5e83f8a546d260c8d93b308
(cherry picked from commit ee41b3a36dcd87f79f3c2a21383a97743530099d)