Clear deferRelaunchUntilPaused flag after activity relaunch is performed

We set deferRelaunchUntilPaused when a relaunch is delayed due to the app
in pausing state, but this flag is not cleared after it's relaunched upon
paused. After that every time the app is paused, it relaunches again.

Change-Id: Ib98956e0760b3c4fe108d93f5c7843e2cbd313de
related-to: b/27834014