Remove un-needed nullables

[NotWoods]

There are various spots where we could hold a reference to a singleton or simply crash if an asset is missing.

[codecov-commenter]

Codecov Report

Merging #13816 into master will increase coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff            @@
##             master   #13816   +/-   ##
=========================================
  Coverage     29.91%   29.92%           
  Complexity     1157     1157           
=========================================
  Files           443      443           
  Lines         17845    17848    +3     
  Branches       2326     2323    -3     
=========================================
+ Hits           5339     5341    +2     
- Misses        12131    12132    +1     
  Partials        375      375           

Impacted Files	Coverage Δ	Complexity Δ
...a/org/mozilla/fenix/browser/BaseBrowserFragment.kt	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
.../java/org/mozilla/fenix/browser/BrowserAnimator.kt	11.76% <0.00%> (ø)	0.00 <0.00> (ø)
...in/java/org/mozilla/fenix/search/SearchFragment.kt	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
.../mozilla/fenix/search/awesomebar/AwesomeBarView.kt	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...mozilla/fenix/searchdialog/SearchDialogFragment.kt	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...tepermissions/SitePermissionsExceptionsFragment.kt	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...mponents/searchengine/FenixSearchEngineProvider.kt	63.96% <0.00%> (+1.80%)	13.00% <0.00%> (ø%)

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