Kill foreground apps when turning off work

While work apps will all be killed as we stop the profile user when
turning off work, this can sometimes take a while. So let's kill the
foreground apps as soon as work profile is being turned off to make
the user experience nicer.

Bug: 27631526
Change-Id: Icdd3799385a98ee531b0b4247e8ede78f6f10da8