a11y: Handle scrollPanel positioning

This CL updates the scroll panel's position to appear next to the
cursor(bottom-right) instead of centered on the screen. It also ensures
proper display even when the cursor is near the screen edges.

Video: http://shortn/_LkskHZGEWH

Bug: b/403700117
Test: AutoclickControllerTest
Flag: com.android.server.accessibility.enable_autoclick_indicator
Change-Id: I7de8ef3a04f95be0fc81c25556e7129e7434ae48