don't run RequestListener callbacks on a Handler unless one is supplied
don't run RequestListener callbacks on a Handler unless one is supplied

sometimes we want to ensure that Future.get() won't return until after
the listener methods have been called and finished.  so now if a caller
wants the listeners run on another thread, the caller needs to pass a
Handler, and must understand that Future.get() will likely return before
the listener has finished running

magazmj/bt-android-utils,kelnos/bt-android-utils