Define SDK/build tool versions in buildSrc

app/build.gradle

@@ -32,13 +32,13 @@ ext {
 }
 
 android {
-    compileSdkVersion 29
-    buildToolsVersion '29.0.3'
+    compileSdkVersion AndroidConfig.compileSdk
+    buildToolsVersion AndroidConfig.buildTools
 
     defaultConfig {
         applicationId "eu.kanade.tachiyomi"
-        minSdkVersion 21
-        targetSdkVersion 29
+        minSdkVersion AndroidConfig.minSdk
+        targetSdkVersion AndroidConfig.targetSdk
         testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
         versionCode 45
         versionName "0.9.2"
@@ -108,8 +108,8 @@ android {
     }
 
     compileOptions {
-        sourceCompatibility = 1.8
-        targetCompatibility = 1.8
+        sourceCompatibility = JavaVersion.VERSION_1_8
+        targetCompatibility = JavaVersion.VERSION_1_8
     }
 
     kotlinOptions {

buildSrc/src/main/kotlin/AndroidConfig.kt

@@ -0,0 +1,6 @@
+object AndroidConfig {
+    const val compileSdk = 29
+    const val minSdk = 21
+    const val targetSdk = 29
+    const val buildTools = "29.0.3"
+}