Closes #39: Wrap UserClearDataEvent in consumable.

[mcomella]

This is an accepted pattern on our team to send single-use events through an Observable.

Pull Request checklist

• [x] This PR includes thorough tests or an explanation of why it does not
  • I've spent too long on #186 and this requires some rethinking about how to test it that I don't want to put in. It's an accepted pattern on our team so I think it's a safe change.
• [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 #256 into master will decrease coverage by 0.02%. The diff coverage is 0%.

@@            Coverage Diff             @@
##             master   #256      +/-   ##
==========================================
- Coverage     35.02%    35%   -0.03%     
  Complexity      371    371              
==========================================
  Files           123    123              
  Lines          3238   3240       +2     
  Branches        507    507              
==========================================
  Hits           1134   1134              
- Misses         2008   2010       +2     
  Partials         96     96

Impacted Files	Coverage Δ	Complexity Δ
...zilla/focus/settings/UserClearDataEventObserver.kt	0% <0%> (ø)	0 <0> (ø)	:arrow_down:
...main/java/org/mozilla/focus/utils/LiveDataEvent.kt	0% <0%> (ø)	0 <0> (ø)	:arrow_down:
...va/org/mozilla/focus/settings/ClearDataFragment.kt	0% <0%> (ø)	0 <0> (ø)	:arrow_down:

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