base: Fix SystemUI crash on Recent App

Issue: SystemUI crash is observed when Context MenuItem is
       selected on the recent apps activity [Long pressing
       two recent apps at same time or Long press, lock and
       unlock the screen]

Fix: Changed the code to make sure that only one context
     menu is active at a time and also dismissed the
     context menu on pause of the RecentsActivity

CRs-Fixed: 539509

Change-Id: Ia50bed7bfad6fde7beeaec37fa6402177eee1d40
Signed-off-by: hany <hany.alsamman@gmail.com>