Large screen screenshots: Update OWNERS

Add a comment mentioning that wesokuhara@ can review large-screen
screenshots code, and remove xiaohuic@ from OWNERS since they don't work
on the code.

Bug: 412421893
Test: None
Flag: EXEMPT OWNERS change only

Change-Id: I19fcf3d8c47f05e91384d4c63575d3f0d1fe3cf0