Fix crashes related to large background bitmap

[eighthave]

load background bitmaps in a custom AsyncTask

In order to support devices that do not have tons of RAM, dynamically size
the background bitmap when loading so only what is needed is stored in RAM.
Also, there is a much stricker limit of memory allocation when operating in
the main UI thread.  More info here:

https://developer.android.com/training/displaying-bitmaps/load-bitmap.html
https://developer.android.com/training/displaying-bitmaps/process-bitmap.html

fixes #4277 https://dev.guardianproject.info/issues/4277

[n8fr8]

Thanks for putting up with my desire for "decoration"

[eighthave]

I also like pretty apps and think it is important, I just think we need to
prioritize and get a really stable release out. The OTR stuff is still pretty
problematic, for example.

[n8fr8]

That's why we're doing incremental releases twice a month now, slowly but surely tracking all these things down.