Fix a FileObserver jni bug.

The path string in FileObserver jni will not be automatically released
in the while loop. Then it will cause too many local refs in the ref
table. When received too many file changed notifications/events, the
process will be killed for "Failed adding to JNI local ref table".
Test code:
=============================================================
import android.app.Activity;
import android.os.Bundle;
import android.os.FileObserver;
import android.util.Log;

import java.io.File;
import java.io.IOException;

public class TestApp extends Activity {

    public static final String TAG="TestApp";
    FileObserver observer;

    @Override
    public void onCreate(Bundle icicle) {
        super.onCreate(icicle);
        setContentView(R.layout.main);

        observer=new FileObserver("/tmp"){
            @Override
            public void onEvent(int event, String path) {
                if(event == FileObserver.CREATE){
                    Log.e(TAG, "File created [" + path + "]");
                }else if(event == FileObserver.DELETE){
                    Log.e(TAG, "File deleted [" + path + "]");
                }
            }
        };
        observer.startWatching();

        File f=new File("/tmp/a.txt");
        for (int i=0;i<300;i++) {
            try {
                f.createNewFile();
            } catch (IOException e) {
            }
            f.delete();
        }
    }

}
================================================
The last 2 lines of the log:
E/dalvikvm(  705): Failed adding to JNI local ref table (has 512
entries)
E/dalvikvm(  705): VM aborting

0 files changed, 0 insertions, 0 deletions