wifi: hotspot2: Passpoint configuration APIs update

- renamed addPasspointConfiguration to addOrUpdatePasspointConfiguration
  to match the implementation of the API
- updated getPasspointConfigurations to return an empty list instead of
  a null when no configuration is installled

Bug: 33587910
Test: frameworks/base/wifi/tests/runtests.sh
Change-Id: Id40185edb08910c01454c44b77957e2d5c0df722