Allow developer to override Material-style modes in picker dialogs

A recent fix prevented developers from specifying Holo-style mode when
displaying a date or time picker dialog. This CL also cleans up unused
code in TimePickerDialog and documents how themeResId will be used.

Adds hidden @TestApi methods for use in CTS tests. These may be made
public later, but it's too late in MR1 for API changes.

Bug: 31586821
Test: Ice2e203983769f1ea1cfa93105eb97b6fa5176b9
Change-Id: I1b7512b7647ddd7ab987beac2c0aef4fe7cc16bc