Fix discoloration in Split screen

[ErickG]

The grey bar used as a divider between the two apps needs to be changed to black instead of grey. This will also change it to black for devices in the Galazy Z series and Galaxy Tab series.

[USBetaModerator2]

Hello,
Thank you very much for your suggestion.
Your suggestion has been communicated to the developer and the development project manager is reviewing the content. After reviewing your suggestions, we will apply them to the official version if your proposal matches our concept.
We appreciate your contribution to the beta program.

Regards,
One UI Beta Team