More logs for Android unsatisfiedLinkError (flutter/engine#53920)

See https://github.com/flutter/flutter/issues/83596#issuecomment-2140605278, there are two cases:
1. IO issue
2. The directory doesn't exist

Narrow down by including in the logs if the directory exists or not.

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

engine/src/flutter/shell/platform/android/io/flutter/embedding/engine/loader/FlutterLoader.java

@@ -207,8 +207,11 @@ public class FlutterLoader {
                             + cpuArch
                             + ", and the native libraries directory (with path "
                             + nativeLibsDir.getAbsolutePath()
-                            + ") contains the following files: "
-                            + Arrays.toString(nativeLibsContents),
+                            + ") "
+                            + (nativeLibsDir.exists()
+                                ? "contains the following files: "
+                                    + Arrays.toString(nativeLibsContents)
+                                : "does not exist."),
                         unsatisfiedLinkError);
                   }
 

engine/src/flutter/shell/platform/android/test/io/flutter/embedding/engine/loader/FlutterLoaderTest.java

@@ -34,6 +34,7 @@ import java.util.concurrent.ExecutorService;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.mockito.ArgumentCaptor;
+import org.mockito.Mockito;
 import org.robolectric.annotation.Config;
 
 @Config(manifest = Config.NONE)
@@ -61,6 +62,25 @@ public class FlutterLoaderTest {
     verify(mockFlutterJNI, times(1)).updateRefreshRate();
   }
 
+  @Test
+  public void unsatisfiedLinkErrorPathDoesNotExist() {
+    FlutterJNI mockFlutterJNI = mock(FlutterJNI.class);
+    ctx.getApplicationInfo().nativeLibraryDir = "/path/that/doesnt/exist";
+    FlutterLoader flutterLoader = new FlutterLoader(mockFlutterJNI);
+
+    Mockito.doThrow(new UnsatisfiedLinkError("couldn't find \"libflutter.so\""))
+        .when(mockFlutterJNI)
+        .loadLibrary();
+    try {
+      flutterLoader.startInitialization(ctx);
+    } catch (UnsupportedOperationException e) {
+      assertTrue(
+          e.getMessage()
+              .contains(
+                  "and the native libraries directory (with path /path/that/doesnt/exist) does not exist."));
+    }
+  }
+
   @Test
   public void itDefaultsTheOldGenHeapSizeAppropriately() {
     FlutterJNI mockFlutterJNI = mock(FlutterJNI.class);