net: conn_mgr: Track ready count ephemerally

Instead of incrementing and decrementing global counter,
just recompute the ready-count from scratch every time
conn_mgr_mon_handle_update is called.

This will simplify the introduction of additional ready count types.

This should have no externally observable impact on the behavior of
conn_mgr.

Signed-off-by: Georges Oates_Larsen <georges.larsen@nordicsemi.no>