Log dlopen errors in opt builds (flutter#41477)

As the Engine uses `dlopen` to find the `libapp.so`, flutter/flutter#59834 can happen which will prevent `dlopen` from finding the binary. In theory this should be mitigated by flutter#9762, but an internal customer observed errors that could be related. 

The additional logging here can help to rule out that hypothesis. For Googlers, see b/276657840 for more details.

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

fml/platform/posix/native_library_posix.cc

@@ -13,8 +13,10 @@ NativeLibrary::NativeLibrary(const char* path) {
   ::dlerror();
   handle_ = ::dlopen(path, RTLD_NOW);
   if (handle_ == nullptr) {
-    FML_DLOG(ERROR) << "Could not open library '" << path << "' due to error '"
-                    << ::dlerror() << "'.";
+    // TODO(jiahaog): Use FML_DLOG:
+    // https://github.com/flutter/flutter/issues/125523
+    FML_LOG(ERROR) << "Could not open library '" << path << "' due to error '"
+                   << ::dlerror() << "'.";
   }
 }