[MBL-782] Update NotificationActivity to RX2

[mtgriego]

📲 What

Update NotificationActivity to RX2

🤔 Why

Getting close y'all

🛠 How

🧠

📋 QA

Make sure the toggling of notification settings still works, even after closing the app

Story 📖

MBL-782

[codecov-commenter]

Codecov Report

Attention: 28 lines in your changes are missing coverage. Please review.

Comparison is base (6c857bd) 74.04% compared to head (456b227) 73.94%.

Files	Patch %	Lines
...m/kickstarter/viewmodels/NotificationsViewModel.kt	69.56%	6 Missing and 22 partials ⚠️

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Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1933      +/-   ##
============================================
- Coverage     74.04%   73.94%   -0.11%     
+ Complexity     1968     1964       -4     
============================================
  Files           337      337              
  Lines         19649    19693      +44     
  Branches       2718     2738      +20     
============================================
+ Hits          14550    14562      +12     
- Misses         3512     3523      +11     
- Partials       1587     1608      +21     

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