Remove unmatched mUserLeaving setting

mUserLeaving is intended to "wrap" a synchronous call (in that
we set to true, do something, set to false). However, this one
spot is setting it to true without unsetting it which is causing
later non-userleaving actions to incorrectly send userleaving
to apps at the wrong times.

Bug: 334038395
Test: mix enter/exit pip + screen off/on repeatedly
Change-Id: I4700faeab4a390e0343513aa4e4adb44dde6f1d9