Make CI functional again (#64)

* Use Ubuntu 20.04 in CI

* Use new SDK number for Android R

Author: mathiascode

.github/workflows/android.yml

@@ -5,7 +5,7 @@ on: [push, pull_request]
 jobs:
   build:
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     steps:
     - uses: actions/checkout@v2

app/build.gradle

@@ -1,7 +1,7 @@
 apply plugin: 'com.android.application'
 
 android {
-    compileSdkVersion 'android-R'
+    compileSdkVersion 30
     defaultConfig {
         applicationId "org.cuberite.android"
         resConfigs "en", "de", "nl", "pt"