a11y: Fix - Correct scroll panel button vertical alignment

Problem: Scroll panel buttons (center, sides, bottom) appeared slightly
misaligned vertically.

Cause: The panel background drawable is 184dp(180+4) tall after we add a
4dp shadow path at the bottom. This shifts the visual shape boundaries
relative to the layout boundaries.

Solution: Move the 184dp height background to the parent, and change the
parent height to 184dp,therefore the background is not squeezed

Screenshot(make the button background to be white for better
visibility):
- before: http://screen/7VbaC8WYVY3D5SD
- after:  http://screen/3xw8EsehZNRCXbH

Bug: b/400964604
Test: AutoclickControllerTest
Flag: com.android.server.accessibility.enable_autoclick_indicator
Change-Id: I8d66fda0c9eb67a2b0a540ab2cd36b5ddf7ffbf1