feat(mobile-next): 导入官方 openim-flutter-demo 固定基线 3.8.3-patch.3 并完成双构建

- 上游: github.com/OpenIMSDK/openim-flutter-demo tag 3.8.3-patch.3
  commit b3dfdb1e8aaeaaf6f0793e10cadd20d5c184a31f
- 并行目录 mobile-next/ 原样导入（含 LICENSE），不覆盖现网 mobile/
- 锁定 Flutter 3.24.5 / Dart 3.5.4 / JDK 17 / Gradle 7.6.3 / AGP 7.3.1
- 实测 Android debug 与 release 构建均成功（见 docs/mobile-next-baseline-import.md）
- 本提交可单独回退：git revert 1db1229（重写前）

Co-authored-by: multica-agent <github@multica.ai>

mobile-next/local_plugin/flutter_openim_live_alert/android/build.gradle

@@ -0,0 +1,41 @@
+group 'io.openim.live.alert.flutter_openim_live_alert'
+version '1.0'
+
+buildscript {
+    repositories {
+        maven { url 'https://maven.aliyun.com/nexus/content/groups/public/' }
+        maven { url 'https://maven.aliyun.com/nexus/content/repositories/jcenter' }
+        maven { url 'https://maven.aliyun.com/nexus/content/repositories/google' }
+        google()
+        mavenCentral()
+    }
+
+    dependencies {
+        classpath 'com.android.tools.build:gradle:4.1.0'
+    }
+}
+
+rootProject.allprojects {
+    repositories {
+        maven { url 'https://maven.aliyun.com/nexus/content/groups/public/' }
+        maven { url 'https://maven.aliyun.com/nexus/content/repositories/jcenter' }
+        maven { url 'https://maven.aliyun.com/nexus/content/repositories/google' }
+        google()
+        mavenCentral()
+    }
+}
+
+apply plugin: 'com.android.library'
+
+android {
+    compileSdkVersion 31
+
+    compileOptions {
+        sourceCompatibility JavaVersion.VERSION_1_8
+        targetCompatibility JavaVersion.VERSION_1_8
+    }
+
+    defaultConfig {
+        minSdkVersion 16
+    }
+}