Update and fix project

[friederbluemle]

Three quick commits here (please let me know if you prefer separate PRs).

Commit 1

Fix test method names: #36 accidentally added two duplicate method names, resulting in the following test failures:

> Task :yearclass:compileDebugUnitTestJavaWithJavac FAILED
yearclass/src/test/java/com/facebook/device/yearclass/YearClassTest.java:102: error: method testGetYearCategoryS7() is already defined in class YearClassTest
  public void testGetYearCategoryS7() {
              ^
yearclass/src/test/java/com/facebook/device/yearclass/YearClassTest.java:110: error: method testTotalRAM() is already defined in class YearClassTest
  public void testTotalRAM() {
              ^
2 errors

Commit 2

Enable a basic GitHub Action CI running ./gradlew build (which also runs the unit tests)

Commit 3

Update the build structure and versions to make if fully compatible with the latest version of Android Studio (3.6.1).

_{^{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!}}