libcutils: use 0 for netlink socket port id

AudioHAL and SoundtriggerHAL is separeted HAL but running in thread with
same process id.
So, if both HAL try to open netlink socket using uevent_open_socket(),
secondly opening socket receives already-in-use error.
To prevent situation, set 0 to socket port id.
By the LINUX man page, "The kernel assigns the process ID to the first
netlink socket the process opens and assigns a unique nl_pid to every
netlink socket that the process subsequently creates."

Bug:163008274
Test: tested by opening netlink socket in AudioHAL and
SoundtriggerHAL both.

Signed-off-by: Gyeongtaek Lee <gt82.lee@samsung.com>
Change-Id: I5f9859e8dac749bf5d0998e825043c9988c202ba