Reuse stash handle alpha animation constants

Taskbar and bubble bar stash handle should use a similar animation for
stash and unstash.
Remove duplicate alpha animation constants and use only the ones defined
in taskbar controller for taskbar and bubble bar.

Bug: 345680453
Flag: EXEMPT refactor
Test: m
Change-Id: I9edf847fc4148bc731051de60a883011e6bfb848