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.

Change-Id: I3ae54aa7d53ff1b81252212973646a95d2d82e8a
Note: This is for low memory devices. (~512MB RAM or less)

Conflicts:
	services/java/com/android/server/am/ProcessList.java