Fix bad cts test

The existing test sent a click message to a nonexistent button
id (0). This happened to work before for no particularly good
reason (because of the way that the old TimePickerDialog was implemented).
It no longer works in L, but that's the fault of the test, not the
widget.

The fix is to click an actual button: TimePickerDialog.BUTTON_POSITIVE.
This change makes the test pass (and makes the test more reasonable).

Issue #17284516 CTS:android.app.cts.TimePickerDialogTest#testOnClick test fails with LMP Release

Change-Id: I3ee068e0c15049857d38fc62cca792fde5244cd7