[battery_plus] Migrate Android part to Kotlin (fluttercommunity#1100)

* Add Kotlin plugin, replace Java plugin file with Kotlin file
* Clean up the code
* Update Flutter dependencies, add release notes
* Fix import for tests

packages/battery_plus/battery_plus/CHANGELOG.md

@@ -1,3 +1,10 @@
+## 2.2.0
+
+- Android: Migrate to Kotlin
+- Android: Bump targetSDK to 33 (Android 13)
+- Android: Update dependencies, build config updates
+- Update Flutter dependencies
+
 ## 2.1.4+1
 
 - Add issue_tracker link.

packages/battery_plus/battery_plus/README.md

@@ -38,5 +38,3 @@ battery.onBatteryStateChanged.listen((BatteryState state) {
   // Do something with new state
 });
 ```
-
-**Important:** As of January 2021, the Flutter team is no longer accepting non-critical PRs for the original set of plugins in `flutter/plugins`, and instead they should be submitted in this project. [You can read more about this announcement here.](https://github.com/flutter/plugins/blob/master/CONTRIBUTING.md#important-note) as well as [in the Flutter 2 announcement blog post.](https://medium.com/flutter/whats-new-in-flutter-2-0-fe8e95ecc65)

packages/battery_plus/battery_plus/android/build.gradle

@@ -8,7 +8,7 @@ buildscript {
     }
 
     dependencies {
-        classpath 'com.android.tools.build:gradle:7.0.2'
+        classpath 'com.android.tools.build:gradle:7.3.0'
     }
 }
 
@@ -20,9 +20,10 @@ rootProject.allprojects {
 }
 
 apply plugin: 'com.android.library'
+apply plugin: 'kotlin-android'
 
 android {
-    compileSdkVersion 31
+    compileSdkVersion 33
 
     defaultConfig {
         minSdkVersion 16
@@ -31,5 +32,8 @@ android {
     lintOptions {
         disable 'InvalidPackage'
     }
+}
 
+dependencies {
+    implementation "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
 }