Extract quick action sheet observer code

[NotWoods]

Move out the session and session manager observers into its own class.

Pull Request checklist

• Quality: This PR builds and passes detekt/ktlint checks (A pre-push hook is recommended)
• Tests: This PR includes thorough tests or an explanation of why it does not
• Changelog: This PR includes a changelog entry or does not need one
• Accessibility: The code in this PR follows accessibility best practices or does not include any user facing features

[codecov-io]

Codecov Report

Merging #4368 into master will decrease coverage by 0.01%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##             master   #4368      +/-   ##
===========================================
- Coverage      6.69%   6.68%   -0.02%     
  Complexity      152     152              
===========================================
  Files           203     204       +1     
  Lines          8494    8493       -1     
  Branches       1189    1188       -1     
===========================================
- Hits            569     568       -1     
  Misses         7886    7886              
  Partials         39      39

Impacted Files	Coverage Δ	Complexity Δ
...a/org/mozilla/fenix/browser/BaseBrowserFragment.kt	0% <ø> (ø)	0 <0> (ø)	⬇️
...uickactionsheet/QuickActionSheetSessionObserver.kt	0% <0%> (ø)	0 <0> (?)
.../java/org/mozilla/fenix/browser/BrowserFragment.kt	0% <0%> (ø)	0 <0> (ø)	⬇️
app/src/main/java/org/mozilla/fenix/ext/Context.kt	16% <0%> (-4%)	0% <0%> (ø)

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