fix: downgrade kotlin & take kotlinVersion from ext

Example/android/build.gradle

@@ -1,13 +1,21 @@
 // Top-level build file where you can add configuration options common to all sub-projects/modules.
 
 buildscript {
+    ext {
+        buildToolsVersion = "30.0.2"
+        minSdkVersion = 21
+        compileSdkVersion = 30
+        targetSdkVersion = 30
+        ndkVersion = "20.1.5948944"
+        kotlinVersion = "1.4.10"
+    }
     repositories {
         google()
         mavenCentral()
     }
     dependencies {
         classpath('com.android.tools.build:gradle:4.2.1')
-        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:1.5.20"
+        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlinVersion"
     }
 }
 
@@ -27,11 +35,3 @@ allprojects {
         maven { url 'https://www.jitpack.io' }
     }
 }
-
-ext {
-    buildToolsVersion = "30.0.2"
-    minSdkVersion = 21
-    compileSdkVersion = 30
-    targetSdkVersion = 30
-    ndkVersion = "20.1.5948944"
-}

TestsExample/android/build.gradle

@@ -7,14 +7,15 @@ buildscript {
         compileSdkVersion = 30
         targetSdkVersion = 30
         ndkVersion = "20.1.5948944"
+        kotlinVersion = "1.4.10"
     }
     repositories {
         google()
         mavenCentral()
     }
     dependencies {
         classpath('com.android.tools.build:gradle:4.2.1')
-        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:1.5.20"
+        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlinVersion"
         // NOTE: Do not place your application dependencies here; they belong
         // in the individual module build.gradle files
     }

android/build.gradle

@@ -1,12 +1,15 @@
 buildscript {
+    ext.safeExtGet = {prop, fallback ->
+        rootProject.ext.has(prop) ? rootProject.ext.get(prop) : fallback
+    }
     repositories {
         google()
         jcenter()
         mavenCentral()
     }
     dependencies {
         classpath('com.android.tools.build:gradle:4.2.2')
-        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:1.5.20"
+        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:${safeExtGet('kotlinVersion', '1.4.10')}"
         classpath "com.diffplug.spotless:spotless-plugin-gradle:5.15.0"
     }
 }
@@ -19,10 +22,6 @@ if (project == rootProject) {
 apply plugin: 'com.android.library'
 apply plugin: 'kotlin-android'
 
-def safeExtGet(prop, fallback) {
-    rootProject.ext.has(prop) ? rootProject.ext.get(prop) : fallback
-}
-
 android {
     compileSdkVersion safeExtGet('compileSdkVersion', 28)
 
@@ -57,6 +56,4 @@ dependencies {
     implementation 'androidx.coordinatorlayout:coordinatorlayout:1.1.0'
     implementation 'androidx.swiperefreshlayout:swiperefreshlayout:1.0.0'
     implementation 'com.google.android.material:material:1.1.0'
-    implementation "androidx.core:core-ktx:1.6.0"
-    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:1.5.20"
 }