Add FlaggedAPI annotation to NSD offload APIs

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