Set up basic CI

.gitlab-ci.yml

+image: registry.gitlab.com/fdroid/ci-images:client-latest
+
+cache:
+  paths:
+    - .gradle/wrapper
+    - .gradle/caches
+
+before_script:
+  - export GRADLE_USER_HOME=$PWD/.gradle
+#  - export ANDROID_COMPILE_SDK=`sed -n 's,.*compileSdkVersion\s*\([0-9][0-9]*\).*,\1,p' app/build.gradle`
+#  - echo y | android --silent update sdk --no-ui --filter android-${ANDROID_COMPILE_SDK}
+
+test:
+  script:
+    - ./gradlew test
+
+after_script:
+    # this file changes every time but should not be cached
+    - rm -f $GRADLE_USER_HOME/caches/modules-2/modules-2.lock
+    - rm -fr $GRADLE_USER_HOME/caches/*/plugin-resolution/