Closes #186: Add SessionRestore

[mcomella]

Ignore code for #39 please.

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)

[codecov-io]

Codecov Report

Merging #257 into master will increase coverage by 0.89%. The diff coverage is 80.85%.

@@             Coverage Diff              @@
##             master     #257      +/-   ##
============================================
+ Coverage        35%   35.89%   +0.89%     
- Complexity      371      395      +24     
============================================
  Files           123      125       +2     
  Lines          3240     3282      +42     
  Branches        507      512       +5     
============================================
+ Hits           1134     1178      +44     
+ Misses         2010     2007       -3     
- Partials         96       97       +1

Impacted Files	Coverage Δ	Complexity Δ
...pp/src/main/java/org/mozilla/focus/MainActivity.kt	0% <0%> (ø)	0 <0> (ø)	:arrow_down:
...zilla/focus/settings/UserClearDataEventObserver.kt	100% <100%> (+100%)	3 <0> (+3)	:arrow_up:
...a/org/mozilla/focus/MainActivityIntentResponder.kt	100% <100%> (+10%)	9 <2> (+2)	:arrow_up:
...rg/mozilla/focus/session/SessionRestorerStorage.kt	100% <100%> (ø)	7 <7> (?)
.../java/org/mozilla/focus/session/SessionRestorer.kt	93.33% <93.33%> (ø)	8 <8> (?)
...main/java/org/mozilla/focus/ext/FragmentManager.kt	0% <0%> (ø)	0% <0%> (ø)	:arrow_down:
...lla/focus/session/VisibilityLifeCycleCallback.java	29.16% <0%> (+4.16%)	4% <0%> (+1%)	:arrow_up:
...rc/main/java/org/mozilla/focus/ScreenController.kt	5.12% <0%> (+5.12%)	2% <0%> (+2%)	:arrow_up:
... and 2 more

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[severinrudie]

@mcomella FYI, I'm on PTO tomorrow. If a short delay is fine, I'll review this on Tuesday.