Implement a generic in-app message banner

[st3fan]

Pull Request checklist

• [ ] This PR includes thorough tests or an explanation of why it does not
• [ ] This PR includes a CHANGELOG entry or does not need one
• [ ] The UI tests are passing after this PR (./gradlew connectedAmazonWebViewDebugAndroidTest)
• [ ] I have considered adding QA labels on the associated issue (not this PR; qa-ready or qa-denied)

[codecov-io]

Codecov Report

Merging #339 (0a78dac) into master (187a1af) will decrease coverage by 0.24%. The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master     #339      +/-   ##
============================================
- Coverage     21.85%   21.60%   -0.25%     
  Complexity      127      127              
============================================
  Files            92       92              
  Lines          1922     1944      +22     
  Branches        298      299       +1     
============================================
  Hits            420      420              
- Misses         1464     1486      +22     
  Partials         38       38              

Impacted Files	Coverage Δ	Complexity Δ
...pp/src/main/java/org/mozilla/focus/MainActivity.kt	0.00% <0.00%> (ø)	0.00 <0.00> (ø)

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