Closes #262: integrate Mozilla gradle plugin

[mcomella]

Pull Request checklist

• [x] This PR includes thorough tests or an explanation of why it does not
  • Nothing to test
• [x] This PR includes a CHANGELOG entry or does not need one
• [ ] The UI tests are passing after this PR (./gradlew connectedAmazonWebViewDebugAndroidTest)
  • Should be unaffected
• [x] I have considered adding QA labels on the associated issue (not this PR; qa-ready or qa-denied)

[codecov-io]

Codecov Report

Merging #263 into master will increase coverage by 0.55%. The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #263      +/-   ##
============================================
+ Coverage        35%   35.55%   +0.55%     
- Complexity      371      396      +25     
============================================
  Files           123      125       +2     
  Lines          3240     3333      +93     
  Branches        507      524      +17     
============================================
+ Hits           1134     1185      +51     
- Misses         2010     2050      +40     
- Partials         96       98       +2

Impacted Files	Coverage Δ	Complexity Δ
...main/java/org/mozilla/focus/ext/FragmentManager.kt	0% <0%> (ø)	0% <0%> (ø)	:arrow_down:
...pp/src/main/java/org/mozilla/focus/MainActivity.kt	0% <0%> (ø)	0% <0%> (ø)	:arrow_down:
...rg/mozilla/focus/session/SessionRestorerStorage.kt	100% <0%> (ø)	7% <0%> (?)
.../java/org/mozilla/focus/session/SessionRestorer.kt	93.33% <0%> (ø)	8% <0%> (?)
...lla/focus/session/VisibilityLifeCycleCallback.java	29.16% <0%> (+4.16%)	4% <0%> (+1%)	:arrow_up:
...a/org/mozilla/focus/MainActivityIntentResponder.kt	94.44% <0%> (+4.44%)	10% <0%> (+3%)	:arrow_up:
...rc/main/java/org/mozilla/focus/ScreenController.kt	5.12% <0%> (+5.12%)	2% <0%> (+2%)	:arrow_up:
...zilla/focus/settings/UserClearDataEventObserver.kt	55.55% <0%> (+55.55%)	3% <0%> (+3%)	:arrow_up:
...main/java/org/mozilla/focus/utils/LiveDataEvent.kt	100% <0%> (+100%)	1% <0%> (+1%)	:arrow_up:

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[mcomella]

@Baron-Severin FYI: I added a commit to fix ./tools/sign_release.sh, whose --test functionality would have broken by this change (because it didn't pass along the "don't validate the build" state to gradle).