FastScroller: Fix to use appropriate index

Fix FastScroller to use appropriate index to determine the
the position of a section while calculating the y coordinates
of thumb. In case of last section, use totalItemCount to determine
next section position.

Change-Id: I89bfd483949b5c14f85303b50380a73b4a4e635e
HeaderViewListAdapter: Correct thrown error names.

When accessing an invalid entry of the list, an IndexOutOfBounds exception is thrown, not an ArrayIndexOutOfBounds exception.

Change-Id: I3cf59faab004fa6391d84f30f280e0c9bd92dc44
Signed-off-by: Taylor H. Perkins <taylorhp@gmail.com>