Disable drag from taskbar while in desktop.

Currently drag from taskbar is broken. It will be fixed once drag from
taskbar to desktop is implemented in b/297921594.

Bug: 323388149
Test: Try to drag an app from taskbar while in desktop, see that app is
not launch and icon goes back to it's place in taskbar.
Flag: NONE

Change-Id: I27e73d8262e14944b34e193397a83f8435813339