Clarifying and stabilizing drag & drop

The existing Shelf DND crashes if profile or app removal happens
while dragging from outside of shelf. This is because in this case
shelf temporary adds a null placeholder enter to the model, and
doesn’t check for nulls in user-removed and app-removed callbacks.

Adding checking for nulls.
In addition, since adding nulls muds the contract between the
shelf view and its model, I’m getting rid of adding null entries
to the model. Nulls are used on the view side, but never leak to
the model.

Because of that, the state of the view (which may have nulls) and the
mode may not match each other during the drag, I introduce storing
app data in views, as a tag. Null tag means a drag placeholder.

Now that the state of the model doesn’t always follow the state
of the shelf via add/remove operations, it makes sense to introduce
set-all/get-all calls in the model, instead of add/remove/get(i)
calls.

Again, all these changes, even though relatively massive, add clarity
into the view/model contract by eliminating strange states where the
model can have nulls in certain positions.

Also fixing a case when removing an app while dragging it happily
creates an icon.

Bug: 20024603
Change-Id: Ie4e617ccf9278708d64ee100265a4858d1227aed