Fix a regression in pausing activity immediately on sleep

At some point during refactoring of ActivityStack, the code to pause the current
activity got deleted. Added back that line of code. Activity will now pause
as soon as the screen is turned off, rather than after 5 seconds (sleep timeout).

Bug: 10632898

Change-Id: If3cc8708d692d29a13dbd8cfd9edda8883b38c2e