frameworks/base: Fix SystemUI crash on Recent Apps list

Issue: SystemUI crash is observed when Context MenuItem "App info"
       is selected on the recent apps activity
       1. Open the rencet apps list
       2. Long press on any one/more recent app(s)
       3. Lock the screen using power key
       4. Unlock the screen using power key
       5. select menu item "App info"
       6. SystemUI process will get crashed.

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

Change-Id: Ia50bed7bfad6fde7beeaec37fa6402177eee1d40