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