samples: prod_consumer: use app_shared.h instead of main.h

Normally main.c file doesn't have a header file, because it doesn't
need to declare any interfaces to other modules.

In this sample, it's better to put the shared variables in one place,
and provide the interfaces to other modules, such as app_a, app_b,
and main.

Signed-off-by: Paul He <pawpawhe@gmail.com>