Improve SDK and implement CMHomeApiManager.

Improve SDK to support new data columns and various features needed for
an extension host to read from cards. Implement CMHomeApiManager that
provides logic for reading and manipulating cards.

Change-Id: Iee98e4eb29e8606ee9a2380d6cd0fa8c3bd8bd01
Change DataCard Priority to be an enum.

To make priority easier to understand and enforce only three specific
values (low, mid and high), make the priority field of DataCard an enum.

Change-Id: Ib22cb59f10b494fb0fbf2bf4acd05a4beeddf9e2