hotspot2: update PasspointConfiguration APIs

Based on the API guideline, use of public variables are discouraged.
So update PasspointConfiguration and its associated classes to use
private variables with public accessor methods.

While there, cleanup unit tests to reduce code duplications.

Bug: 34627062
Test: frameworks/base/wifi/tests/runtests.sh
Change-Id: I6ea45bbcf03aec01c187425a66094fad6098d75d