API: Fix parameter validation in setnotificationtimestamp

This was broken in I7a3d7b6e, when the ApiPageSet parameters stopped
being returned by getAllowedParams() and so by extractRequestParams().
Although it would be broken differently if they had been.

Change-Id: I4b6ec21fd7b7c932856546df1ccad574d996db1f
Merge "API: Fix parameter validation in setnotificationtimestamp"