am: Account for time changes when showing recent activities

[Procedures]
1. Go to Settings->Date&Time
   Set time backwards, eg. 2016.6.15 -> 2016.6.10

2. Tap recent apps button to show recent activities.
   There will be no activities shown.

RM-290

Change-Id: Ifd860c0d08fac49f9d3c8eac590b6db9e6afb09d