staging: android: lowmemorykiller: implement task's adj rbtree [Hong-Mei Li]

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>