Bluetooth: Mesh: Fix Proxy Privacy parameter support

According to section 6.7, upon connection Proxy Server shall determine
value of Proxy Privacy parameter (sections 6.5 and 7.2.2.2.6). Depending
on that, it will either send Secure Network Beacon or Private Beacon to
Proxy Client, but never both.

Proxy Privacy parameter is determined by GATT Proxy, Node Identity
states and their private counterparts (section 7.2.2.2.6). Since
non-private and private states are mutually exclusive, it is enough to
only check either Private GATT Proxy state or Private Node Identity
state of any known subnet for which the state is currenty enabled.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>