passpoint-r2: change return type of getMatchingOsuProviders

This change is required to show signal strength with OSU provider name
on UI

Bug: 119514793
Test: ./frameworks/base/wifi/tests/runtests.sh
Test: tested with R2 AP for connection
Signed-off-by: Ecco Park <eccopark@google.com>

Change-Id: I4abb69ffad3ccd855db935e738d1896b263da147