Revert "Add shadow to assistant handles"

This reverts commit c445bbf3.

Reason for revert: Causes memory issues on the GPU (~50mb -> ~70mb)

Test: Memory usage goes back down to 53mb with this revert.
https://sponge.corp.google.com/invocation?tab=Logfile+Index&id=7643bf50-41bb-45a7-8c93-d0ddb1f82a04&searchFor=
(last file, search for com.android.systemui#anon-and-swap-avg-median)

Bug: 141187457

Change-Id: Ia48eca3f95097562771abc3a523bd32b74f2d443