gitlab-ci: ensure compile platform is installed

Avoid broken builds when fdroidclient gets out of sync with
ci-images/client.

`emulator -no-skin` no longer does anything, but give a warning.

.gitlab-ci.yml

@@ -7,6 +7,8 @@ cache:
 
 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:
@@ -53,7 +55,7 @@ connected24:
   variables:
     AVD_SDK: "24"
   script:
-    - emulator64-arm -avd fcl-test-$AVD_SDK -no-skin -no-audio -no-window &
+    - emulator64-arm -avd fcl-test-$AVD_SDK -no-audio -no-window &
     - ./tools/wait-for-emulator
     - adb shell input keyevent 82 &
     - export EXITVALUE=0