[ottf] hide FreeRTOS internals from concurrency API
[ottf] hide FreeRTOS internals from concurrency API

This further cleans up the OTTF concurrency API using a forward
declaration to a FreeRTOS function pointer to hide FreeRTOS internals
from OTTF users.

Signed-off-by: Timothy Trippel <77eacc84f9735d3cbb55a271710be269fc3a5818@google.com>

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