provide correct firstAnnouncement prop type
provide correct firstAnnouncement prop type

flag=none

test plan:
  - specs pass

qa risk: low

Change-Id: Ib4b764ced6a09b0630388557ff636aca79deac2a
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/274444
Reviewed-by: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>
QA-Review: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>
Product-Review: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>

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