Update getCheckedItemPositions() documentation

getCheckedItemPositions()'s documentation previously mentioned the 
returned SparseBooleanArray was only containing checked item positions 
(i.e. get(int position) always returning true). In practice, this is 
wrong, because getCheckedItemPositions() returns mCheckStates which 
basically contains all key-value mappings that have been used. As a 
consequence if the item at position p is checked and unchecked, the 
returned SparseBooleanArray will contain a (p, false) mapping.

Another option could be to delete all keys having a false value.

Change-Id: I15f8fc2a1ba3f1501af126c272b827402cfd8b14