Update background-support app's memory policy and reorganize some code

Update background-support app's memory policy and reorganize some code

[Issue#] N/A
[Problem] There's a limitation on hidden applications number(current 15) which is kept by
apps' /proc/[pid]/oom_adj values. Least recently used app will have the highest oom_adj
value, when hidden apps number limitation is reached, the app will be killed. However
background-support enabled app shouldn't be one of this kind, it shouldn't be count as
a hidden app when goes to background.
[Cause] N/A
[Solution] Make background-support enabled app active when goes to background by using
sysman's API. oom_adj value will be updated accordingly by these APIs. Move
backgroun-support check logic from RunnableWidgetObject to ViewLogic.

Change-Id: I29ef92153e345c31bd50d4c034f44fa6fced5e33