Update jni Android Gradle scripts to avoid Groovy space-assignment deprecations

[ancient0328]

The Android Gradle scripts for pkgs/jni and pkgs/jni_flutter currently use Groovy space-assignment / generated setter syntax in several places, for example:

pkgs/jni/android/build.gradle:

• group 'com.github.dart_lang.jni'
• version '1.0'
• namespace 'com.github.dart_lang.jni'
• compileSdk 35
• ndkVersion flutter.ndkVersion
• minSdk 21

pkgs/jni_flutter/android/build.gradle:

• group 'com.github.dart_lang.jni_flutter'
• version '1.0'
• namespace 'com.github.dart_lang.jni_flutter'
• compileSdk 35
• minSdk 21

With recent Gradle versions, these forms emit deprecation warnings. Gradle's upgrade guide states that generated property setter methods for Groovy space-assignment syntax are deprecated and will be removed in Gradle 10, and recommends explicit assignment such as propertyName = value.

A minimal remediation should be limited to Android Gradle files and should not alter Dart, Java, Kotlin, native runtime, public API, or package dependency semantics.

Reference: https://docs.gradle.org/current/userguide/upgrading_version_8.html#groovy_space_assignment_syntax

[liamappelbe]

We need to check if this is an issue for JNIgen's generated gradle stubs too: pkgs/jnigen/lib/src/tools/android_sdk_tools.dart, _gradleGetClasspathStub and _gradleGetSourcesStub

Also affects a bunch of examples:

• pkgs/jni/example/android/app/build.gradle
• pkgs/jnigen/android_test_runner/android/app/build.gradle
• pkgs/jnigen/example/in_app_java/android/app/build.gradle
• pkgs/jnigen/example/kotlin_plugin/android/build.gradle
• pkgs/jnigen/example/kotlin_plugin/example/android/app/build.gradle
• pkgs/jnigen/example/maven_libs_groovy/example/android/app/build.gradle
• pkgs/jnigen/example/notification_plugin/android/build.gradle
• pkgs/jnigen/example/notification_plugin/example/android/app/build.gradle
• pkgs/jnigen/example/pdfbox_plugin/android/build.gradle
• pkgs/jnigen/example/pdfbox_plugin/example/android/app/build.gradle