[Lens] Remove delay in applying drag styles on workspace panel

[wylieconlon]

There is a small but noticeable animation delay in the background-color of the main workspace panel, as seen in this video:

I talked with @cchaos about potential solutions, and we haven't settled on one but we should try these in order:

1. Identify if there are performance issues related to drag+drop that are causing slow animations
2. Switch to the faster animation delay from EUI
3. Remove the transition on the background color

Other solutions might be possible as well.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)