bump to drone image 28

.drone.yml

@@ -1,6 +1,6 @@
 pipeline:
   test:
-    image: nextcloudci/android:android-26
+    image: nextcloudci/android:android-28
     commands:
       # uncomment gplay for Gplay, Modified only
       - sh -c "if [ '$FLAVOUR' != 'Generic' ]; then sed -i '/.*com.google.*/s/^.*\\/\\///g' build.gradle; fi"
@@ -29,7 +29,7 @@ pipeline:
       - LD_LIBRARY_PATH=/usr/lib/x86_64-linux-gnu/:/opt/android-sdk-linux/tools/lib64/gles_mesa/
 
   lint:
-    image: nextcloudci/android:android-26
+    image: nextcloudci/android:android-28
     commands:
       # needs gplay
       - sed -i '/.*com.google.*/s/^.*\\/\\///g' build.gradle