Fixes for HeaderViewListAdapter.

Null header and footer, as used by CTS tests, were no longer supported.
Added a static empty list to avoid repetitive null tests in that case.
Fixed getItem/getItemId/getView to handle corner cases.
Changed ListAdapter isEnabled documentation for invalid positions.

http://b/issue?id=2527753
Change-Id: I55e5bf21cb0673d906caa7c669987a6ae869d90f