Replace JNI string conversion workarounds with direct NewStringUTF and GetStringUTFChars to fix FindClass crash on background threads.

[leiyue123]

问题

SafeConvertToJString 和 SafeConvertToStdString 使用 static 懒初始化缓存 java/lang/String 类，通过 FindClass 查找。如果首次调用发生在后台线程（如 ThreadPoolExecutor），FindClass 可能因为线程的 ClassLoader 上下文不完整而返回 null，触发 CheckJNI abort（java_class == null）。

相关 issue：#3104、#3175

根因

原实现为了规避 Android 4.x Dalvik 时代 NewStringUTF 对 4 字节 UTF-8 字符的崩溃问题，采用了绕路方案：通过 FindClass → GetMethodID → NewObject 来构造 Java String。但 FindClass 在不同线程上行为不可靠。

libpag 当前 minSdk 为 21（Android 5.0+），运行时已切换为 ART，该 NewStringUTF bug 已在 ART 中修复。

修复

• SafeConvertToJString：直接用 env->NewStringUTF() 替代 Java 构造器绕路
• SafeConvertToStdString：直接用 GetStringUTFChars/ReleaseStringUTFChars 替代 Java getBytes() 回调

改动量：2 文件，+5 -40 行。消除所有 static FindClass 依赖，在任何线程调用都安全。

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.33%. Comparing base (417f38d) to head (84f81bf).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3566      +/-   ##
==========================================
- Coverage   81.34%   81.33%   -0.02%     
==========================================
  Files         653      653              
  Lines       77243    77243              
  Branches    21919    21919              
==========================================
- Hits        62837    62825      -12     
- Misses       9975     9981       +6     
- Partials     4431     4437       +6     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.