libbinder_ndk: toString never goes to empty strings.

This needs more follow-up, but we should show values directly.

Before, an array of a single valid PFD would look like an empty array
"[]". Now, we distinguish an empty array "[]" from "[fd:3]".

Fixes: 260177075
Change-Id: I7c8ea833e999c8cda24db92fa1ac84b86a51602f
Test: N/A
diff --git a/libs/binder/ndk/include_cpp/android/binder_to_string.h b/libs/binder/ndk/include_cpp/android/binder_to_string.h
index 6a25db2..2a00736 100644
--- a/libs/binder/ndk/include_cpp/android/binder_to_string.h
+++ b/libs/binder/ndk/include_cpp/android/binder_to_string.h
@@ -160,7 +160,7 @@
 template <typename _T>
 std::string ToString(const _T& t) {
     if constexpr (details::ToEmptyString<_T>::value) {
-        return "";
+        return "<unimplemented>";
     } else if constexpr (std::is_same_v<bool, _T>) {
         return t ? "true" : "false";
     } else if constexpr (std::is_same_v<char16_t, _T>) {
@@ -176,9 +176,11 @@
         return t;
 #ifdef HAS_NDK_INTERFACE
     } else if constexpr (std::is_same_v<::ndk::SpAIBinder, _T>) {
-        return (t.get() == nullptr) ? "(null)" : "";
+        std::stringstream ss;
+        ss << "binder:" << std::hex << t.get();
+        return ss.str();
     } else if constexpr (std::is_same_v<::ndk::ScopedFileDescriptor, _T>) {
-        return (t.get() == -1) ? "(null)" : "";
+        return "fd:" + std::to_string(t.get());
 #endif
 #ifdef HAS_STRING16
     } else if constexpr (std::is_same_v<String16, _T>) {