staging: android: lmk: check free memory when tasks switch to background

Add more free-memory check point.
This patch trigger low memory killer to scan free memory when a process is moved
to background task groups.

Change-Id: If50eef2fb2c05f57ab808f95a9408204058f4f9b
Signed-off-by: Hong-Mei Li <a21834@motorola.com>
Reviewed-on: http://gerrit.pcs.mot.com/557973
Tested-by: Jira Key <jirakey@motorola.com>
Reviewed-by: Yi-Wei Zhao <gbjc64@motorola.com>
SLTApproved: Yi-Wei Zhao <gbjc64@motorola.com>
Submit-Approved: Jira Key <jirakey@motorola.com>

Conflicts:
	include/linux/sched.h