Closes #219: When exiting fullscreen, defer removing windowBackground.

[mcomella]

Pull Request checklist

• [x] This PR includes thorough tests or an explanation of why it does not
• [x] This PR includes a CHANGELOG entry or does not need one
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• [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 #242 into master will increase coverage by 0.15%. The diff coverage is 50%.

@@             Coverage Diff              @@
##             master     #242      +/-   ##
============================================
+ Coverage     34.67%   34.83%   +0.15%     
  Complexity      370      370              
============================================
  Files           123      122       -1     
  Lines          3247     3264      +17     
  Branches        507      513       +6     
============================================
+ Hits           1126     1137      +11     
- Misses         2023     2026       +3     
- Partials         98      101       +3

Impacted Files	Coverage Δ	Complexity Δ
...org/mozilla/focus/architecture/ViewModelFactory.kt	0% <ø> (ø)	0 <0> (ø)	:arrow_down:
...pp/src/main/java/org/mozilla/focus/ext/LiveData.kt	87.5% <0%> (-5.84%)	0 <0> (ø)
...pp/src/main/java/org/mozilla/focus/MainActivity.kt	0% <0%> (ø)	0 <0> (ø)	:arrow_down:
.../java/org/mozilla/focus/browser/BrowserFragment.kt	0% <0%> (ø)	0 <0> (ø)	:arrow_down:
...java/org/mozilla/focus/browser/BrowserViewModel.kt	89.28% <82.35%> (-10.72%)	6 <2> (+2)

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[mcomella]

In debugging an unrelated issue, I discovered I could move the code to the BrowserFragment which better encapsulates it than keeping it in the MainActivity. I made the move and force-pushed my code. Over to @Baron-Severin to verify this approach is acceptable.

[mcomella]

Closing this PR: sorry, Severin, I found a better solution. Apparently the stuttering issue was being caused by us translating the WebView while it was exiting fullscreen to make room for the toolbar. We can more simply fixing the toolbar by disabling this animation, which was already pretty janky anyway.