DownloadCache plugin causes SIGSEGV crash on Android

[gentledepp]

When using MvxImageView with local images (i.e. local file paths), from time to time the application crashes with a SIGSEGV error and without any stacktrace or a stacktrace that contains "SKBitmapRef..."

This bug seems to exist since DownloadCache 3.5.1 - so many users downgraded to 3.5.0.

I was finally able to track down the error and will post a PR sool with a fix.

The bug is also prominent on stackoverflow:
http://stackoverflow.com/questions/30924141/native-crash-sigsegv-on-android-5-0-with-xamarin-mvvmcross

It is kinda hart to reproduce this bug, as it is GC dependent and does not happen on a totally regular basis, but if you have a ListView with a bunch of MvxImageViews which all bind to a local image and you scroll up and down for a while (range from 5 seconds to 3 minutes on our devices), you'll get that crash. Trust me.