issue/1927-motion-event-illegal-argument

Fix #1927 - I could never reproduce the problem, but I'm pretty sure this bug is the same as the one [described here](https://github.com/chrisbanes/PhotoView/issues/31) which would impact the ReaderPhotoViewerActivity. Resolved the problem by having that activity and ReaderPostPagerActivity use a custom ViewPager which catches the IllegalArgumentException.