passpoint-r2: add @systemApi for Passpoint APIs

Making Passpoint APIs @Systemapi is required for Setup Wizard to use.

Bug: 119514793
Test: ./frameworks/base/wifi/tests/runtests.sh
Change-Id: I3aadfdb30295de8a2997f154705af0b1b27f3a5e
Signed-off-by: Ecco Park <eccopark@google.com>