After much consternation and testing, it became apparant that there
was nothing really wrong with our PendingIntent setup or notification
setup. The only problem was the rapidness with which the notification
was being updated. There is something about rapidly updated notificaitons
which makes it not possible to hit the action button. This explains why
the button _would indeed work sometimes_, because the user may have hit
it just in that sweet spot.
This change increases the time between progress events from 100ms to 500ms
which seems to do the job on my Moto X 2nd gen. Perhaps this can be changed
to a larger number if required. When it was set to 300 ms, it _mostly_ worked,
but there was still a few times where hitting the button wouldn't work at all.
Fixes#652.
b3f79da341