SurfaceFlinger: tune number of buffers for low activity layers

Tune number of buffers indicating whether a layer is low activity
or not to 2 accommodate more cases of low activity layers.

Bug: 140948091
Test: 1) Set the static wallpaper
2) Go to Settings->About Phone -> scroll all the way down
3) Move device to Zone1 and launch settings page
4) Move device to Non Zone1 without touch and wait for 1-2 secs and observe

Change-Id: I6a06fa4945843a8d991463c30cb0e6db68f99365