Migrate from android::String isEmpty to empty [aapt]

This empty method is different from the old one - it aligns with
std::string definition.

Bug: 295394788
Test: make checkbuild
Change-Id: I1900f2285ee3c3b1eb57d2cef3990896a64dae01
diff --git a/libs/androidfw/AssetManager2.cpp b/libs/androidfw/AssetManager2.cpp
index b1ef4e5..e7a788e 100644
--- a/libs/androidfw/AssetManager2.cpp
+++ b/libs/androidfw/AssetManager2.cpp
@@ -956,13 +956,13 @@
     const auto& assets = GetApkAssets(step.cookie);
     log_stream << "\n\t" << prefix << ": " << (assets ? assets->GetDebugName() : "<null>")
                << " #" << step.cookie;
-    if (!step.config_name.isEmpty()) {
+    if (!step.config_name.empty()) {
       log_stream << " - " << step.config_name;
     }
   }
 
   log_stream << "\nBest matching is from "
-             << (last_resolution_.best_config_name.isEmpty() ? "default"
+             << (last_resolution_.best_config_name.empty() ? "default"
                                                    : last_resolution_.best_config_name)
              << " configuration of " << last_resolution_.best_package_name;
   return log_stream.str();