Reduce impact of RenderThread in system_server

- Don't try to persist snapshots during window animations. It can
wait and doing it later while decrease the load during window
animations, leading to less jank.
- Lower RT priority to 130 from system_server. It's only used for
background things.

Bug: 37631016
Bug: 36631902
Change-Id: I4fc21b53f257d0194a7f64c3064756ab2426ff17