a11y: Update ScrollPanel UI

Update scroll panel UI to match the spec, focusing on the panel and
arrow container shape. Details like color, shadow, opacity, and hover
effects will be addressed in the next CL.

Video:
 - Before: http://shortn/_Vx8VNyxHA0
 - After:  http://shortn/_Qc0eSd9rNP

Bug: b/400964604
Test: AutoclickControllerTest
Flag: com.android.server.accessibility.enable_autoclick_indicator
Change-Id: I906bdffa976c034b9a8af80874e8bb7fa95bd246