Closes #267: use viewLifecycleOwner instead of adding/removing observers

[mcomella]

Pull Request checklist

• [x] This PR includes thorough tests or an explanation of why it does not
  • Code swap, no need to add tests
• [x] This PR includes a CHANGELOG entry or does not need one
• [x] The UI tests are passing after this PR (./gradlew connectedAmazonWebViewDebugAndroidTest)
• [x] I have considered adding QA labels on the associated issue (not this PR; qa-ready or qa-denied)

[codecov-io]

Codecov Report

Merging #268 into master will increase coverage by 0.09%. The diff coverage is 0%.

@@             Coverage Diff              @@
##             master     #268      +/-   ##
============================================
+ Coverage     35.89%   35.99%   +0.09%     
  Complexity      395      395              
============================================
  Files           125      125              
  Lines          3282     3273       -9     
  Branches        512      509       -3     
============================================
  Hits           1178     1178              
+ Misses         2007     1998       -9     
  Partials         97       97

Impacted Files	Coverage Δ	Complexity Δ
...rg/mozilla/focus/home/NavigationOverlayFragment.kt	0% <0%> (ø)	0 <0> (ø)	:arrow_down:
.../java/org/mozilla/focus/browser/BrowserFragment.kt	0% <0%> (ø)	0 <0> (ø)	:arrow_down:

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