get rid of “image provider”s and Images.cs
load drallo icons off the ui thread
remove DiskCacheProvider
adapt android code (get rid of DiskCacheProvider usage)
Merge branch 'DA-358' into unstable

* DA-358:
  adapt android code (get rid of DiskCacheProvider usage)
  remove DiskCacheProvider
  load drallo icons off the ui thread
  get rid of “image provider”s and Images.cs