Return a port from bindToPort() in IpSecService

When binding to a random port, the port number was not being
returned, so the user is incorrectly receiving a port of zero.
Return the port number that was ultimately bound.

Bug: 67662580
Test: Ran CTS test on aosp_angler-eng
Change-Id: I22f4e5cdc83d43dc453788889990feb34e451bd3