Allow current active live wallpaper app to bypass app switching protection

For example, if user clicks home button and clicks the live wallpaper
to read something, we allow the current live wallpaper to bypass app switching
protection to show activity.

Bug: 183838549
Test: Install a live wallpaper app and set it as wallpaper, click home button, click live wallpaper, live wallpaper can start an activity successfully.
Change-Id: I140e6dc832d4de949f44b489b9c1c441cadefb7b