MBL-807: ActivityFeed migrated to RXJava2

[Arkariang]

📲 What

• Migration RXJava2 + ViewModel, take a look at the engineering plan if any doubt
• android.defaults.buildfeatures.buildconfig (deprecated) moved to gradle.build

| | |

📋 QA

• Poke around the ActivityFeed

Story 📖

MBL-799

[codecov-commenter]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (ac0203c) 74.09% compared to head (e1ee1cb) 74.06%.

Files	Patch %	Lines
...s/rx/transformers/IncrementalCountTransformerV2.kt	57.14%	3 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1929      +/-   ##
============================================
- Coverage     74.09%   74.06%   -0.03%     
  Complexity     1967     1967              
============================================
  Files           336      337       +1     
  Lines         19634    19642       +8     
  Branches       2716     2716              
============================================
+ Hits          14547    14548       +1     
- Misses         3499     3506       +7     
  Partials       1588     1588              

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