wifi: managed Passpoint configurations in "Saved Networks" page

Instead of going through WifiTracker to retrieve saved network
configurations (including both WiFi networks and Passpoint provider
configurations), use the WifiManager's APIs to retrieve the
configurations directly, to avoid any unnecessary complexities.

Also update to use the appropriate APIs for configuration removal
based on the configuration type.

Bug: 34207710
Test: manual test by removing a Passpoint configuration from
      "Saved Networks" page.

Change-Id: I549cc484d8825b6c38895428c1b7941390bb5f1f