aws-ota: Fix demo memory access error

`MQTTAgent_Init()` must be called with arguments that have a lifetime
that extends to the entire run of the program for the `pNetworkBuffer`
and `pTransportInterface` parameters as the memory addresses are used
later on internally by the MQTT agent.

This commit ensures the arguments passed to `MQTTAgent_Init()` for
those parameters have a lifetime that extends to the entire run of
the program by making them static variables.

Signed-off-by: Hugues Kamba-Mpiana <hugues.kambampiana@arm.com>