enforce permissions on diagnostics API

The permission decorator prevents compilation if an app is missing the
ENTER_TRADE_IN_MODE permission and is accessing the API, however we
still need to enforce the permission at runtime on the service side.

Bug: 397154502
Flag: com.android.tradeinmode.flags.trade_in_mode_2025q4
Test: th
Change-Id: I21a6dd0eb5dc17d7cc1218c6f85cdf3af80c73ed