Align example Android NDK and Java target with the Flutter template

Two divergences from what `flutter create` generates, both surfaced as
build warnings:

- ndkVersion was pinned to 27.0.12077973 while connectivity_plus,
  device_info_plus, flutter_background, flutter_webrtc and jni all
  require 28.2.13676358. Use `flutter.ndkVersion` (the template's own
  reference, currently 28.2.13676358) so this tracks the SDK instead of
  drifting again.
- Java source/target compatibility was VERSION_1_8, which now warns
  "source/target value 8 is obsolete and will be removed in a future
  release". The template uses VERSION_17.

kotlinOptions.jvmTarget moves to 17 alongside it; leaving it at 1_8 would
fail with inconsistent JVM-target compatibility.

Both warnings are gone from the build output and the APK still builds.

Author: hiroshihorie

example/android/app/build.gradle

@@ -8,15 +8,15 @@ plugins {
 android {
     namespace = "io.livekit.example"
     compileSdk = flutter.compileSdkVersion
-    ndkVersion = "27.0.12077973"
+    ndkVersion = flutter.ndkVersion
 
     compileOptions {
-        sourceCompatibility = JavaVersion.VERSION_1_8
-        targetCompatibility = JavaVersion.VERSION_1_8
+        sourceCompatibility = JavaVersion.VERSION_17
+        targetCompatibility = JavaVersion.VERSION_17
     }
 
     kotlinOptions {
-        jvmTarget = JavaVersion.VERSION_1_8
+        jvmTarget = JavaVersion.VERSION_17
     }
 
     defaultConfig {