This is caused by the Voice features being set to null. The fix in
Android Marshmallow is to make onGetFeaturesForLanguage return an
empty HashSet object. This does not work for eSpeak because:

  1.  eSpeak was overriding onGetVoices and initializing each Voice's
      features to null;

  2.  the bug is still present on Lollipop.

Thus, the fix here is two-fold:

  1.  make onGetVoices use onGetFeaturesForLanguage;

  2.  make onGetFeaturesForLanguage return an empty HashSet, so that
      eSpeak does not crash on Lollipop either.