mod/apcontext: Refactor context handling and add unit tests

Refactors the AP context module to store the configuration pointer directly
in the module context (`ctx.config`). This simplifies access throughout the
module and avoids repeated calls to `fwk_module_get_data()`.

Also adds a safeguard to prevent `wait_on_notifications` from being
decremented below zero.

Includes unit tests for init, start, and notification handling.

Signed-off-by: Leandro Belli <leandro.belli@arm.com>