[Passpoint] New APIs for HomeSP OtherHomePartners

Following feedback from API council, modified the OtherHomePartners
get/set APIs back to be hidden, and created new APIs that use
a Collection instaed of an array. Updated CTS tests.

Bug: 169330230
Test: atest HomeSpTest PasspointConfigurationTest
Change-Id: I454679196dabebdf0649a544b9b0c135800f5941