Stop jumping to workspace from hotseat on quick drop.

Regression occurred in ag/1315317.

In that change, Launcher#enterSpringLoadedDragMode() was moved from
the end of startDrag() to the end of onDragStart(). This changed the
sequence of events from:

beginDragShared() --> DragController#startDrag() --> onDragStart -->
dropTarget.onDragOver() --> Launcher#enterSpringLoadedDragMode()

to:

beginDragShared() --> DragController#startDrag() --> onDragStart -->
Launcher#enterSpringLoadedDragMode() --> onDragOver()

Basically, the key difference is that onDragOver() is called after
enterSpringLoadedDragMode(). This matters, because onDragOver()
returns early if Workspace#transitionStateShouldAllowDrop() returns
false, which it does during the spring loaded transition. This meant
that onDragOver() didn't update the layout to be Hotseat, and thus
the current workspace page was used as default, since that is what
was set in onDragEnter(). To fix it, I've copied the logic in
onDragOver() that checks to see if the drag is over the hotseat first
to a new method that onDragEnter() now calls as well.

Bug: 31245181
Change-Id: Ie758921b25fc96516a09c32bdc9706da941522df