Bugfix/customproxy

[ashishkumar468]

Description (required)
Added a CustomProxy class, the instance of which overrides the invocationHandler() method which returns appropriate values for different datatypes instead of null for all of them. See https://stackoverflow.com/questions/52083338/expected-to-unbox-a-string-primitive-type-but-was-returned-null
Fixes #2737 App crashed with NPE in UploadPresenter due to null object returned by Proxy InvocationHandler

Tests performed (required)

Tested {ProdDebug} on {Pixel} with API level {27}.

[codecov-io]

Codecov Report

Merging #2738 into master will increase coverage by 0.02%.
The diff coverage is 31.57%.

@@            Coverage Diff            @@
##           master   #2738      +/-   ##
=========================================
+ Coverage    2.72%   2.75%   +0.02%     
=========================================
  Files         267     268       +1     
  Lines       12801   12816      +15     
  Branches     1137    1141       +4     
=========================================
+ Hits          349     353       +4     
- Misses      12426   12437      +11     
  Partials       26      26

Impacted Files	Coverage Δ
...va/fr/free/nrw/commons/upload/UploadPresenter.java	12.63% <100%> (+1.58%)	⬆️
...in/java/fr/free/nrw/commons/utils/CustomProxy.java	7.14% <7.14%> (ø)

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