Remove incorrect call to Window.addFlags with a system flag

The same flag is being (correctly) added in onCreate, so on top of being wrong this was also completely unnecessary.

Bug: 368579654
Test: atest NotificationAccessConfirmationActivityTest
Flag: EXEMPT bugfix

(cherry picked from commit e0510a62)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:824d46f313eb83c1d4a8ad48585dabf6763f5645)
Merged-In: I539fca77a1d136bdd0e4892993020884430b8a4e
Change-Id: I539fca77a1d136bdd0e4892993020884430b8a4e