xtos: move sof/lib/agent.h to application interface

The SOF agent.h interface is a system agent that is implemented on top
of SOF audio task scheduling interface. An agent task is added to the
low-latency scheduler to monitor health of the system. The current
implementation is actually RTOS agnostic and can run on top of both
Zephyr and XTOS.

Some RTOSes offer a lower level watchdog interface to implement system
monitoring. Previously agent.h was considered as the abstraction point,
onto which RTOS specific implementations can be hooked in.

This patch moves agent.h back to application interface. In the future, a
more low-level agent hooking into a watchdog system (either hardware
watchdog directly, or software abstraction like Zephyr's task_wdt) can
be added on the side, and enabled on a per target basis. The audio
scheduler level SOF agent will continue to be available as an option,
and can be used with all RTOS'es.

Link: #9015
Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>