Update Android Gradle plugin and Wrapper

[friederbluemle]

This updates the Android Gradle plugin to latest version 3.6.1. Gradle Wrapper was updated to the matching version 5.6.4.

It also includes some whitespace fixes, and enables a basic GitHub Actions based CI (running ./gradlew build).

Example of a successful build: https://github.com/friederbluemle/aamirror/runs/466118365

_{^{Once approved, please use a normal GitHub merge (i.e. NO rebase/squash merge) to integrate the commit(s) from the PR head branch. The changes are broken up into meaningful, atomic commits, and my branch should already be up-to-date with the latest base branch. If it isn't, or if you want me to change anything, please let me know, and I will update the branch as soon as possible. Thank you!}}

[friederbluemle]

@slashmax - Please review and merge when you have a minute. Thanks! :)

[sebaKwiat]

@friederbluemle can you provide apk file it might be useful for others looking for latest version of the app.