Fix a bug that the RecentTask list of other’s may show up to non-primary users.

When switching to a newly created user, the user may face this bug when he opens the RecentTask screen.
A possible bug scenario is described as follows:
A user id of a removed user may be recycled when created a new user.
However, mRecentTasks is not correctly controlled so that old information may still remain and be possibly mapped to wrong user.
This patch prevents this bug by explicitly removing old information in mRecentTasks when removing existing user.

Change-Id: I1874dbd604598a5d740ae1e034981e21214c15c6
Signed-off-by: Eunae Kim <eunae.kim@lge.com>