Fix live wallpaper stays on AOD mode

After unlock the device, the live wallpaper status stay on AOD mode.

WallpaperConnect does not disconnect while users existing wallpaper
app by entering home or pressing back key.

Disconnect WallpaperConnection in onStop().

Video:
https://drive.google.com/file/d/13Ek4ZSyjNHeUcAv-nh9uT8daYHN4i7rm/view

Bug: 158434793
Test: Manual
Change-Id: I0eac6ff1bf6ba9be9d51987a330c1b1bd9c2bcd4