a11y: ScrollPanel: Update panel's color to match the spec

1. Update color to match the spec.
2. Add shadow(4px down) to the scrollPanel.
3. Update the exit-button width and height from 50 to 48 to match the
   spec.

screenshot: http://screen/72duHYUqbC9xfVK

Bug: b/400964604
Test: AutoclickControllerTest
Flag: com.android.server.accessibility.enable_autoclick_indicator
Change-Id: I52d42377cb500d9d62465c5a744af2244edca3d8