aws-iot: Refactor ota_agent_task redundant code

The function `otaAppCallback` is called upon an event such as a new
file block being received, or the OTA image being activated.
The function then redundantly checks the OTA state via `OTA_GetState`,
which reduces code clarity via duplication including redundant
calls to the ota update's active and inactive hooks.
This fix has been tested via passing Arm's internal CI.

Signed-off-by: Reuben Cartwright <Reuben.Cartwright@arm.com>