如果SYSTEM_ALERT_WINDOW权限可用，则返回true。
*从Q开始，在低ram手机上禁用SYSTEM_ALERT_WINDOW。

vendor/mediatek/proprietary/packages/apps/MtkSettings/src/com/android/settings/Utils.java

public static boolean isSystemAlertWindowEnabled(Context context) {
        // SYSTEM_ALERT_WINDOW is disabled on on low ram devices starting from Q
        ActivityManager am = (ActivityManager) context.getSystemService(Context.ACTIVITY_SERVICE);
        return true/*!(am.isLowRamDevice() && (Build.VERSION.SDK_INT >= Build.VERSION_CODES.Q))*/;
    }