Launcher3 : checkItemPlacement fixes

This patchset addresses two bugs :

1) Incorrect casting of key causes mismatches
2) On hotseat items 2 and above, the occupied hashmap is populated
with the CONTAINER_HOTSEAT key, however if its a new item and its required
cell is not already been taken, the code falls through to the workspace checks.
Make sure hotseat items are treated separately.

Change-Id: I3f22e46940f62a223453bd05314eae2a642c899a