Reduce total number of background apps for low memory devices

Due to memory constraints, we anyway cannot fit the default number
of BG apps. So instead of killing background apps to
free up memory when launching, start with an already
reduced background set of apps.

Another side-effect is that launch latencies also improve.
This change is for low memory devices. (512MB RAM or less)

Change-Id: I9de0cb1734c0a386fc47cde71723e5c16825f5a3
ActivityManagerService: Penalise large apps going to background

Large applications (PSS more than a given threshold) when closed are
penalised with higher ADJ. Now, they will be more prone to be killed by
lowmemorykiller during memory pressure levels. This ensures better app
concurrency and improved subsequent latencies as it leaves some more room
for other smaller bg apps to be alive

Change-Id: I448fa12f55370949b22075595cd75b9c29e5d4b5