android 按指定包名返回版本号

有个需求，想判断手机里是否安装指定的app，没找到现成的代码，参考Delphi代码实现了一个，当返回空串时，表示没有安装app。

function GetVersionString(APackageName:string): string;
//uses Androidapi.JNI.GraphicsContentViewText,Androidapi.Helpers,androidapi.jni.javatypes;
var
  PackageInfo: JPackageInfo;
  PackageManager: JPackageManager;
  AppContext: JContext;
begin
  AppContext := TAndroidHelper.Context;
  if AppContext <> nil then
  begin
    PackageManager := AppContext.getPackageManager;
    if PackageManager <> nil then
    begin
      try
        PackageInfo := AppContext.getPackageManager.getPackageInfo(StringToJString(APackageName), 0);
      except
        Exit(string.Empty);
      end;

      if PackageInfo <> nil then
         Exit(JStringToString(PackageInfo.versionName));

    end;
  end;
  Result := string.Empty;
end;