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: manual
Flag: EXEMPT bugfix

(cherry picked from commit e0510a62)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:4f4d00f42271df5a2727cac373412dd0114076cf)
Merged-In: I539fca77a1d136bdd0e4892993020884430b8a4e
Change-Id: I539fca77a1d136bdd0e4892993020884430b8a4e