Issue #107: Fix getting `ListView`

An error was reintroduced by the merge where the `Context` for
`BaseballCardAdapter` (formerly `CheckedCursorAdapter`) was cast to a
`ListActivity`. With the addition of an `ActionBar`, this cast is no
longer valid. The `ListView` must be obtained by calling
`findViewById()` on the `Activity` rather than calling `getListView()`
on a `ListActivity`.