Fix unsupported api usage in NumberPicker

There is unsupported access to the divider. Exposing the
drawable will be tricky as the widget counts on the drawable
being a certain shape and have certain attributes. However we can
easily expose the thickness as a new attribute and document that
you can customize the divider color via XML.

Bug: 117520784
Test: atest NumberPickerTest (with new test)
Change-Id: Ib7a65c055df3b1f68c8327943fa887c89f57e00c