frontend: Change executed method name in frontend callbacks

Changed the IFrontendMultipleActionAsyncCallback,
IFrontendMultipleQueryAsyncCallback and IFrontendActionAsyncCallback
interfaces callback method from 'Executed' to 'executed' to fit
Java conventions

Change-Id: Id6b75edbf47490d3b964cbf153989d5af5dcc39e
Signed-off-by: Tal Nisan <tnisan@redhat.com>