staging: android: lowmemorykiller: implement task's adj rbtree

Based on the current LMK implementation, LMK has to scan all processes to
select the correct task to kill during low memory.
The basic idea for the optimization is to :
queue all tasks with oom_score_adj priority, and then LMK just selects the
proper task from the queue(rbtree) to kill.

performance improvement:
the current implementation: average time to find a task to kill : 1004us
the optimized implementation: average time to find a task to kill: 43us

Change-Id: I4dbbdd5673314dbbdabb71c3eff0dc229ce4ea91
Signed-off-by: Hong-Mei Li <a21834@motorola.com>
Reviewed-on: http://gerrit.pcs.mot.com/548917
SLT-Approved: Slta Waiver <sltawvr@motorola.com>
Tested-by: Jira Key <jirakey@motorola.com>
Reviewed-by: Yi-Wei Zhao <gbjc64@motorola.com>
Submit-Approved: Jira Key <jirakey@motorola.com>

Conflicts:

	drivers/staging/android/Kconfig
	drivers/staging/android/lowmemorykiller.c