應用壓到後臺，關閉應用某個權限後，應用無法正常使用

 在 application中註冊，可以統一管理activity 的生命週期

這樣可以用於判斷應用是否前後臺。

 應用壓到後臺，關閉應用某個權限後，再回到應用無法正常使用，解決辦法是回到頁面後關閉頁面，讓應用重啓。

默認isKilled = true。當應用正常啓動時，置爲false。

class ActivityLifeCallback implements Application.ActivityLifecycleCallbacks {

    private int frontActivityCount;

    private ArrayList<Activity> mActivityList = new ArrayList<>();

    private long lastPauseTime;

    /**
     * 用於保存當前Activity的實例<br>
     * 每次跳轉或銷燬一個Activity頁面<br>
     * 此對象都會被重新賦值，故並不會出現Activity泄漏
     */
    private Activity mCurrentActivity;

    /**
     * 此回調在Activity的OnCreate方法裏調用 super.onCreate(savedInstanceState)時執行
     * 以下方法皆是如此
     */
    @Override
    public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
        if (activity instanceof SplashActivity) {
            if (mActivityList.size() == 0 || activity.isTaskRoot()) {
                MainApplication.isKilled = false;
            }
        }
        if (MainApplication.isKilled) {
            //進程被殺死，重啓app
            restartApp(activity);
            return;
        }

        mCurrentActivity = activity;
        if (mActivityList.contains(activity)) return;
        mActivityList.add(activity);
    }

    @Override
    public void onActivityStarted(Activity activity) {

    }

    @Override
    public void onActivityResumed(Activity activity) {
        frontActivityCount++;
        //每次當Activity恢復時，mCurrentActivity對象都會被賦予新的值
        if (mCurrentActivity == activity) return;
        mCurrentActivity = activity;
    }

    @Override
    public void onActivityPaused(Activity activity) {
        frontActivityCount--;
        lastPauseTime = System.currentTimeMillis();
    }

    @Override
    public void onActivityStopped(Activity activity) {
    }

    @Override
    public void onActivitySaveInstanceState(Activity activity, Bundle outState) {

    }

    @Override
    public void onActivityDestroyed(Activity activity) {
        mActivityList.remove(activity);
    }

    Activity getCurrentActivity() {
        return mCurrentActivity;
    }

    public boolean isBackground() {
        return frontActivityCount == 0;
    }

    //清空任務棧
    void clearTask() {
        while (mActivityList.size() > 0) {
            Activity activity = mActivityList.remove(0);
            if (activity != null) activity.finish();
        }
    }

    public boolean isFront() {
        return frontActivityCount > 0;
    }

    private static void restartApp(Context context) {
        Intent launch = context.getPackageManager().getLaunchIntentForPackage(BuildConfig.APPLICATION_ID);
        if (launch == null) return;
        launch.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK | Intent.FLAG_ACTIVITY_NEW_TASK);
        context.startActivity(launch);
        if (context instanceof Activity) {
            ((Activity) context).overridePendingTransition(0, 0);
        }
        System.exit(0);
    }
}