Properly manage JniStringCache lifecycle and clear on unload.

The `JniStringCache` singleton is now initialized using `std::call_once` and a global pointer. A `JNI_OnUnload` function is added to clear the cache and delete the singleton instance, ensuring that global JNI references are released when the library is unloaded.

With this change we successfully pass hwasan:
https://android-build.corp.google.com/builds/abtd/run/L01400030017696946

Bug: 443493398
Flag: EXEMPT BUGFIX
Change-Id: I5474a64ec7146b638094f792c8f2bb47254c0449