Add CI with GitHub Actions

[domdomegg]

Description (required)

Depends on #4076

Adds CI with GitHub actions. It runs much faster (7 minutes vs. 19 minutes when you include time to boot the agent) and integrates with GitHub more closely. Also makes it easier for people to run CI on forks.

This check has passed here: https://github.com/domdomegg/apps-android-commons/actions/runs/404423357

NB: this only does the CI part, not the CD part that Travis also does for us at the moment.

[maskaravivek]

We will also need to include a script for automatic alpha releases. As of now, it's integrated into our Travis flow. Ref: #1056

Edit: We currently do it using ./gradlew publishProdReleaseApk. We will just have to make sure that a bunch of encrypted environment variables, Keystore etc is available when this command is run.

[codecov-io]

Codecov Report

Merging #4078 (d02c368) into master (b24ed1c) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #4078   +/-   ##
=========================================
  Coverage     10.60%   10.60%           
  Complexity      476      476           
=========================================
  Files           343      343           
  Lines         12875    12875           
  Branches        997      997           
=========================================
  Hits           1366     1366           
  Misses        11441    11441           
  Partials         68       68           

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[misaochan]

Thanks a lot @domdomegg .

[domdomegg]

Ah my bad, I think this still depends on updating Gradle which requires moving to JSON service credentials unfortunately. :(

Probably need to revert until that's sorted :(