Closes #297: Set default overlay visible value to false

[dnarcese]

On session restore, the isToolbarScrollEnabled value was not set correctly to true because the overlay was not visited first, and that is when the value changes. Setting isNavigationOverlayVisible default to false is the fix.

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
• [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)

[mcomella]

It looks like lint failed. Severin made partial progress in this PR: https://github.com/mozilla-mobile/firefox-echo-show/pull/291

I'll take a look at the errors and, assuming this PR doesn't change anything, I think I can just force-merge it.

[mcomella]

@dnarcese I noticed you didn't complete the checklist items. In particular, we have no UI tests on CI so can you make sure to run them locally?

[codecov-io]

Codecov Report

Merging #299 into master will increase coverage by 0.09%. The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #299      +/-   ##
============================================
+ Coverage     21.32%   21.41%   +0.09%     
  Complexity      125      125              
============================================
  Files            91       91              
  Lines          1876     1877       +1     
  Branches        293      293              
============================================
+ Hits            400      402       +2     
  Misses         1438     1438              
+ Partials         38       37       -1

Impacted Files	Coverage Δ	Complexity Δ
...rg/mozilla/focus/toolbar/BrowserAppBarViewModel.kt	100% <100%> (ø)	4 <0> (ø)	:arrow_down:
...pp/src/main/java/org/mozilla/focus/ext/LiveData.kt	100% <0%> (+6.66%)	0% <0%> (ø)	:arrow_down:

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