Fix bug where two items could occupy same grid cell in hotseat.

When long pressing on an app to start, a drag would start on the
Workspace but mDragInfo would not be cleared since onDropCompleted
is not called in this case.

Solution is to set mDragInfo to null in onDragEnd.

Bug: 72206125
Change-Id: I2b9a1563c80e591d946a44f4e949b71f7b423a00