main/devicestate: Prevent duplicate registration of device state providers

Currently, the device state provider API will allow you to register a
device state provider with the same case insensitive name more than
once. This could cause strange issues, as the duplicate device state
providers will not be queried when a device's state has to be polled.
This patch updates the API such that a device state provider with the
same name as one that has already registered will be rejected.

Change-Id: I4a418a12280b7b6e4960bd44f302e27cd036ceb2