[Bugfix] Fix the wrong lastBackgroundTime in UidRecord

Test:A process call into updateOomAdjLocked in background
which will set its uidRec.lastBackgroundTime = SystemClock.uptimeMillis();
and then in the idleUidsLocked() method it will compare the
lastBackgroundTime with the elapsedRealtime which may cause a
process which just go into background a few seconds ago become
idle state and then be killed by this

Change-Id: I419a555ef2f6490c26290b19fc86756fde293551
Signed-off-by: lihuan3 <lihuan3@xiaomi.com>