Move all desktop mode intial bounds logic to `DesktopTasksController`

Currently there are two intial bounds calcultions, one in
`DesktopModeWindowDecorViewModel` and another in `DesktopTasksController`.
Previously the calculation in `DesktopModeWindowDecorViewModel` was needed
for the animation to enter desktop mode. However this dependancy has been
removed and thus there is no need to calculate in
`DesktopModeWindowDecorViewModel` specifically as the logic already exits
in 'DesktopTasksController' where it is untimately needed.

Fixes: 334056753
Test: atest WMShellUnitTests:DesktopTasksControllerTest,
WMShellUnitTests: DesktopModeWindowDecorViewModelTests
Change-Id: Ib623ada68add20a234235472e718cf0b3984e33b