[litertlm] fix a Windows JVM issue with absl::flat_hash_map

runtime/engine/BUILD

@@ -47,7 +47,6 @@ cc_library(
     deps = [
         ":engine_interface",
         ":engine_settings",
-        "@com_google_absl//absl/container:flat_hash_map",
         "@com_google_absl//absl/functional:any_invocable",
         "@com_google_absl//absl/log:absl_log",
         "@com_google_absl//absl/status",

runtime/engine/engine_factory.h

@@ -18,10 +18,10 @@
 #include <cstddef>
 #include <memory>
 #include <string>
+#include <unordered_map>
 #include <utility>
 #include <vector>
 
-#include "absl/container/flat_hash_map.h"  // from @com_google_absl
 #include "absl/functional/any_invocable.h"  // from @com_google_absl
 #include "absl/log/absl_log.h"  // from @com_google_absl
 #include "absl/status/status.h"  // from @com_google_absl
@@ -209,10 +209,17 @@ class EngineFactory {
              }},
         }) {}
 
-  absl::flat_hash_map<EngineType, Creator> registry_;
+  // Use std::unordered_map instead of absl::flat_hash_map to avoid Windows
+  // build/linker issues across DLL boundaries (see cl/403158423 and
+  // b/508692203). std::unordered_map can also be faster when the number of
+  // elements is low.
+  std::unordered_map<EngineType, Creator> registry_;
+
   // Map of preferred engine types for each backend. The first available
   // (registered) engine type in the list will be selected by CreateDefault().
-  absl::flat_hash_map<Backend, std::vector<EngineType>> preferred_engines_;
+  // Use std::unordered_map for consistency with registry_ and to avoid similar
+  // Windows issues.
+  std::unordered_map<Backend, std::vector<EngineType>> preferred_engines_;
 };
 
 // Helper struct to register an engine type with the EngineFactory.