wifi: hotspot2: initial implementation of PasspointConfiguration

Only support nodes under HomeSP and Credential subtree that are used
by Hotspot 2.0 Release 1.

These classes are used for data exchanges between the app and the system
service (WifiService).  All the business logics and validations will be
done on the service side (WifiService).

Bug: 31813706
Test: frameworks/base/wifi/tests/runtests.sh

Change-Id: I6b08f52dd7b1a7b337899ab80f11d062c8e2f532