SL-14992 Replaced LLAPRFile's isExist with LLDirUtil's fileExists in fmodstudio

This particular case of LLAPRFile crashes due to thread issues (and if it doesn't it might be affecting some other apr call due to using default pool).
Function is not opening the .dsf file in question and LLAPRFile won't ensure that file exists till the end of the function, it just checks that file exists at a given moment. No point to overcomplicate things by adding thread safe pool, so replaced with dirutil.

indra/llaudio/llaudioengine_fmodstudio.cpp

@@ -804,7 +804,7 @@ bool LLAudioBufferFMODSTUDIO::loadWAV(const std::string& filename)
         return false;
     }
 
-    if (!LLAPRFile::isExist(filename, NULL, LL_APR_RPB))
+    if (!gDirUtilp->fileExists(filename))
     {
         // File not found, abort.
         return false;