1.frameworks/base/packages/SystemUI/src/com/android/systemui/statusbar/phone/PhoneStatusBar.java

//add devin 定义观察者
    final private ContentObserver screenshotShowObserver = new ContentObserver(mHandler) {
        @Override
        public void onChange(boolean selfChange) {
            boolean isShow = Settings.System.getInt(mContext.getContentResolver(), Settings.System.SCREENSHOT_BUTTON_SHOW, 1) == 1;
            ButtonDispatcher screenshotButton = mNavigationBarView.getScreenshotButton();
            screenshotButton.setVisibility(isShow ? View.VISIBLE : View.GONE);
        }
    };
//add end
//add devin 注册观察者
        mContext.getContentResolver().registerContentObserver(
                Settings.System.getUriFor(Settings.System.SCREENSHOT_BUTTON_SHOW), true,
                screenshotShowObserver);
//add end

2.Settings.System.SCREENSHOT_BUTTON_SHOW 默认值的做法请参考添加自定义默认系统属性

3.在packages/apps/Settings/res/xml/display_settings.xml添加一个swith开关控件,在string资源中添加screenshot_show

<SwitchPreference
    android:key="screenshot_show"
    android:title="@string/screenshot_show" />

4.在packages/apps/Settings/src/com/android/settings/DisplaySettings.java 添加代码实现

//定义
    private static final String KEY_SCREENSHOT_SHOW = "screenshot_show";
    private SwitchPreference mScreenshotPreference;
//注册onCreat 中注册
    mScreenshotPreference = (SwitchPreference) findPreference(KEY_SCREENSHOT_SHOW);
    boolean isShow = Settings.System.getInt(resolver, Settings.System.SCREENSHOT_BUTTON_SHOW, 1) == 1;
    if(mScreenshotPreference!=null){
        mScreenshotPreference.setChecked(isShow);
        mScreenshotPreference.setOnPreferenceChangeListener(new OnPreferenceChangeListener() {
            @Override
            public boolean onPreferenceChange(Preference preference, Object newValue) {
            boolean isOn=(Boolean)newValue;
            Settings.System.putInt(mContext.getContentResolver(),Settings.System.SCREENSHOT_BUTTON_SHOW,
                isOn ? 1 : 0);
            return true;
            }
        });
    }