Maintain legacy signatures

The signature field is deprecated, but must still be populated. The
thought was that for APEX files [which is a new concept and thus
could use the new way to handle signatures] we could remove this
capability, but, it turns out Play relies on it.

Change-Id: I7096567082d4f798be1d3e6007122750453665ae
Fixes: 130827182
Test: Manual.
Test: Create sample app that dumped the signatures of an APEX
Test: Note that the signatures before this change were 'null' and set to something after this change