Null pointer tests removed from HeaderViewListAdapter.

The mHeaderViewInfos and mFooterViewInfos lists were sometimes tested for nullity
and sometimes not. Should these values be null, some conditionnal code was actually
flawed in getView() and getItem() because of too strong 'and' conditionnals.

These lists are created upon declaration in ListView and are then shared and hence
cannot be null. Null tests were hence removed. getView() and getItem() get simpler
and work as before.

Change-Id: I279e482730ce6148a559d15a9863afc8f6b67780