passpoint-r2: add validate function for R2 configuration

Bug: 118506846
Test: ./frameworks/base/wifi/tests/runtests.sh
Test: live test with Passpoint R2 service provider AP
Change-Id: I40d2363f2d88733ed02d145cf04332d781f10a8d
Signed-off-by: Ecco Park <eccopark@google.com>