Only add providers if there are none already there. Refreshing cards will add back any that aren't already there.

Change-Id: Iad02e23d13b23b410a3fc9a97afa73e165f85873
Update cards in place if the DashClock extension publishes an update.

Change-Id: Ic9c20e7e650d39a2006530ff243e3058c795e60e