Correctly apply checkstyle plugin

app/build.gradle

@@ -1,4 +1,5 @@
 apply plugin: 'com.android.application'
+apply plugin: 'checkstyle'
 
 android {
     compileSdkVersion 27
@@ -24,6 +25,20 @@ android {
     }
 }
 
+task checkstyle(type: Checkstyle) {
+    configFile = rootProject.file('config/checkstyle/checkstyle.xml')
+
+    source 'src/main/java'
+    include '**/*.java'
+    exclude '**/gen/**'
+
+    classpath = files()
+}
+
+checkstyle {
+    toolVersion = '8.17'
+}
+
 dependencies {
     implementation fileTree(dir: 'libs', include: ['*.jar'])
     implementation 'com.android.support:appcompat-v7:27.1.1'

build.gradle

@@ -29,13 +29,6 @@ allprojects {
             url 'https://igniterealtime.org/repo/'
         }
     }
-
-    apply plugin: 'checkstyle'
-
-    checkstyle {
-        configFile = new File(rootConfigDir, 'checkstyle.xml')
-        toolVersion = '8.10'
-    }
 }
 
 task clean(type: Delete) {