Make "simple" a "button type", not a "color"

Summary:
Ref M1476. Currently, setColor('simple') is meaningful. Instead, setButtonType('simple').

Depends on D18047.

Test Plan: Looked at UI examples, Phame, Auth. Notifications mooted by D18047.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D18048