Support wallpaper on secondary displays.(1/N)

Extends WallpaperConnection:mEngine, mapping an engine to a display.
Handling display change events for add or remove wallpaper on secondary
display.
Only attach wallpaper on accessible display, usually WallpaperService is
third party app and cannot access private display.

Bug: 115486823
Test: atest ActivityManagerMultiDisplayTests
Test: atest WallpaperManagerTest

Change-Id: Idb6063c3cf4c8c5b854676666615e3df4e6d65f4