Support wallpaper on secondary displays.(2/N)

Modify wm struct to let DisplayContent has its own WallpaperController.
The wallpaper target would be decided for each display.
Remove SET_WALLPAPER_MAY_CHANGE flag from WindowSurfacePlacer, its not
used anymore.

Bug: 115486823
Bug: 118645630
Bug: 118662630
Test: Manual test with Chromecast.
Test: atest WallpaperControllerTests
Test: atest WmTests
Test: atest ActivityManagerMultiDisplayTests

Change-Id: I5ec1320ff0500ce3f7b9429a594bc44b7267cc5f