remove drawables targeted at android < 9 since minSdk is 9
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
Merge branch 'eighthave-fix-crashes-related-to-large-background-bitmap'
add new low-res versions of default background