cam-service: Check the received timestamp from the client

Currently the init and start message rely on the client timestamp
to set the state timer, but this timestamp is not checked against
the current timestamp from the cam-service.

Add a check to ensure the received timestamp is in the past,
compared to the current timestamp of the cam-service.

Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>