hotspot2: hide validate method for Passpoint configuration

The validate method is used by Framework only, no need to expose
to app developers. So hide it.

Bug: 35756298
Test: make -j32
Change-Id: I15ccae472f30d7ae968816479f6ea35d90248fe0
Merged-In: I15ccae472f30d7ae968816479f6ea35d90248fe0