Adjust docs around obscured flags and filter touches API

Only one of FLAG_WINDOW_IS_OBSCURED or FLAG_WINDOW_IS_PARTIALLY_OBSCURED
is ever applied (not both). Updated docs around
setFilterTouchesWhenObscured() to clarify that touches on the view whose
window is obscured *at the touched location* are filtered.

Change-Id: Icaac3373c27d8a79bb0ba0c303d9b61d9e79c6a3
Fix: 185132725
Fix: 162501914
Test: TH passes