These APIs were released via SDK extension but they need to have
FlaggedAPI annotation to avoid being released by quarterly release.
Note that this CL does not contain udc-extended-api change.
Once com.android.net.flags.register_nsd_offload_engine_api is marked as
released API, APIs with this annotations should be visible in
udc-mainline-prod API .txt files.
Bug: 301713539
Test: TH
Change-Id: I32b58ec64c1f599f48bc89b189ed24ac1717a1d7
ce23d8467e