For #1481. Use androidx runner in feature-prompts.

[dector]

Issue #1481

• Enable includeAndroidResources and enableUnitTestBinaryResources for feature-prompts module.
• Use AndroidJUnit4 as a test runner (from AndroidX Test Ext).

Complexity

Easy (★☆☆)

Pull Request checklist

• Quality: This PR builds and passes detekt/ktlint checks (A pre-push hook is recommended)
• Tests: This PR includes thorough tests or an explanation of why it does not
• Changelog: This PR includes a changelog entry or does not need one
• Accessibility: The code in this PR follows accessibility best practices or does not include any user facing features

[codecov]

Codecov Report

Merging #3395 into master will decrease coverage by 0.11%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #3395      +/-   ##
============================================
- Coverage     81.44%   81.33%   -0.12%     
- Complexity     2968     3324     +356     
============================================
  Files           400      411      +11     
  Lines         13585    14526     +941     
  Branches       1944     2064     +120     
============================================
+ Hits          11064    11814     +750     
- Misses         1750     1868     +118     
- Partials        771      844      +73

Impacted Files	Coverage Δ	Complexity Δ
...nents/browser/storage/sync/PlacesHistoryStorage.kt	97.82% <0%> (-0.22%)	18% <0%> (-2%)
...ava/mozilla/components/feature/pwa/ext/Activity.kt
.../java/mozilla/components/support/ktx/kotlin/Map.kt
...mozilla/components/support/ktx/kotlin/ByteArray.kt
...ozilla/components/support/ktx/android/view/View.kt
...components/support/ktx/android/view/MotionEvent.kt
...ava/mozilla/components/support/ktx/java/io/File.kt
...java/mozilla/components/support/ktx/kotlin/Char.kt
...ponents/feature/pwa/AbstractWebAppShellActivity.kt
...ts/support/ktx/android/content/res/AssetManager.kt
... and 47 more

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