Fix loading of images in contributions list

[maskaravivek]

Description (required)

Fixes #2892

What changes did you make and why?

Earlier, ContributionViewHolder was trying to set the image url on Schedulers.io() thread. The fix was to use AndroidSchedulers.mainThread() instead.

Also, I have re-added LruCache which was removed in #2906. So now, in an app session, a thumbnail would be fetched only once for a filename.

Tests performed (required)

Tested on prod debug API 27.

Screenshots showing what changed (optional - for UI changes)

[codecov-io]

Codecov Report

Merging #2932 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master   #2932      +/-   ##
=========================================
- Coverage    3.69%   3.69%   -0.01%     
=========================================
  Files         246     246              
  Lines       12215   12219       +4     
  Branches     1083    1083              
=========================================
  Hits          451     451              
- Misses      11730   11734       +4     
  Partials       34      34

Impacted Files	Coverage Δ
.../free/nrw/commons/di/CommonsApplicationModule.java	0% <ø> (ø)	⬆️
.../commons/contributions/ContributionViewHolder.java	0% <0%> (ø)	⬆️

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[ashishkumar468]

Thanks for the PR @maskaravivek. Solves the issue for me