CI & Cache for Gradle/Android workflows

[kirillt]

We need to speed up our CI builds. Right now, it can take up to an hour.

Example provided by @mdrlzy
https://github.com/coil-kt/coil/blob/d0644b9a5a96627a09d5ac724b8a4fd6578fea6e/.github/workflows/ci.yml#L66

- uses: actions/cache@v2
  with:
    path: |
      ~/.gradle/caches
      ~/.gradle/wrapper
    key: ${{ runner.os }}-${{ hashFiles('**/*.gradle*') }}-${{ hashFiles('**/gradle/wrapper/gradle-wrapper.properties') }}-${{ hashFiles('**/buildSrc/**/*.kt') }}