Relaxed WallpaperManager API assertion

This CL relaxes the assertion to only verify the APIs query
Context#getDisplayId. We don't check if the context is display
context because we may change these APIs to provide the window
metrics of a specific DA.

Bug: 178717030
Test: atest StrictModeTest
Change-Id: I34413463eb1eb675871df41f262bce9ae3fb4edb