Add poultney, pomini, santie to WallpaperManager(Service) OWNERS

Currently WallpaperManager has only two owners. This CL adds three folks working actively on the area.

Bug: N/A
Test: N/A, only a change in an OWNERS file
Ignore-AOSP-First: team is working on qpr/master
Change-Id: I504d90aaaf4ddf20a491dddbfe239215584fbaac