Separator in AbsListView should not be clickable

In ListAdapter, isEnabled (int position) returns true if the
item at the specified position is not a separator. (A
separator is a non-selectable, non-clickable item). So we should
not set it to clickable in accessibility node info when the item
is disabled.

Bug: 140017328
Test: After the fix, talkback focus will skip the separator in
linear navigation.

Change-Id: Id0a76a2f93b8af730dab1a808dc24a0ae24bcee0