Added auto-fill support for Spinner.

Spinner is a special kind of list-value field because its auto-fillable
values are not currently present in the assist structure: this change
adds a getAutoFillOptions() to fill that void, and implements the
auto-fill APIs on Spinner.

It also fixes RadioGroup.getAutoFillValue() - it should return the index
of the child, not its resource id.

Bug: 33550221
Test: CtsAutoFillServiceTestCases (with new tests) pass
Change-Id: I625e7dd705fade56fda490fcd3c4446c0c245ee1