Trim task thumbnail within MAX_RECENT_BITMAPS.

Task thumbnails can be hold as much as length of mRecentTasks because
freeLastThumbnail only works when any task is added and it might cause
the system heavy.

Free lastThumbnail when it reuses exist taskRecord also to keep
thumbnails within MAX_RECENT_BITMAPS.

Change-Id: I932a16c20acb3d8ead2a238ba221a0fa19484b9a