nuplayer: looping playback only if it was running

Audio buffer filling in renderer is still happening, though pause was
issued. As a result, EOS was reported irrespective of renderer state.

If nuplayer driver receives playback complete after stopped, it will
again be activated, which appears stop is not taking effect.

Add a check for driver state to allow looping only if it was in running
state.

Change-Id: Ic9f8eac635a774cd805b3978fab640d73ae35744
CRs-Fixed: 958311