passpoint-r2: add OSU flag for WifiConfiguration

It is to indicate the OSU network for suppressing sign-in notification
as captive-portal when user triggers OSU procedure.

Bug: 120149912
Test:  ./frameworks/base/wifi/tests/runtests.sh
Test: test with R2 AP for verifying that sign-in pop up doesn't happen.
Change-Id: Ib00e418471fbbc7c511af8773ff305d2a27363e6
Signed-off-by: Ecco Park <eccopark@google.com>