获取APK第一次安装在本机的时间

PackageInfo 新增 firstInstallTime 和 lastUpdateTime 可以查看一个软件的安装和上次更新时间，这比直接获取APK路径查看文件时间更可靠，对于付费的私有路径未root的机型无权限获取的。

    /**
     * The time at which the app was first installed.  Units are as
     * per {@link System#currentTimeMillis()}.
     */
    public long firstInstallTime;

    /**
     * The time at which the app was last updated.  Units are as
     * per {@link System#currentTimeMillis()}.
     */
    public long lastUpdateTime;