當Low memory時，Activity被銷毀了，但是Fragment重複產生

Reference:

http://juniperphoton.net/2017/03/27/jie-jue-viewpager-fragment-de-hui-fu-wen-ti/
http://y-anz-m.blogspot.tw/2013/07/viewpager-fragment.html
http://www.voidcn.com/article/p-weicgtjm-kv.html

Issue description:

當Low memory時，process被系統刪除，Activity被銷毀了，但是Fragment還在，導致Fragment重疊，造成開啟app的時間不斷增加

使用 adb shell dumpsys activity top 指令
可以發現 fragment數量不停累加

Solution:

在Activity被銷毀前，先將正在使用的fragment object儲存起來
下次開啟app後，再將前一次使用的fragment object (現在用不到的) 刪除掉

Code snippet:

    @Override
    protected void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        // Store the current used fragments
        if (mFragmentAdapter != null) {
            getSupportFragmentManager().putFragment(outState, KEY_FRAG_NAME1, mFragmentAdapter.getFragment(0));
            getSupportFragmentManager().putFragment(outState, KEY_FRAG_NAME2, mFragmentAdapter.getFragment(1));
        }
    }

    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
.................
        // Remove previous unused fragments
        if (savedInstanceState != null) {
            mFragment1 = getSupportFragmentManager().getFragment(savedInstanceState,
                    KEY_FRAG_NAME1);
            mFragment2 = getSupportFragmentManager().getFragment(savedInstanceState,
                    KEY_FRAG_NAME2);

            if(mFragment1 != null || mFragment2 != null){
                FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
                transaction.remove(mFragment1 );
                transaction.remove(mFragment2 );
                transaction.commit();
            }
        }
...............
    }