At last a question I can answer! 😉 I use Cleartune (on Android) and really like it. You can set it to different transpositions (e.g. Bb), produce a drone at a particular pitch, or use it as a tuner. It’s very responsive and the display is laid out beautifully clearly. And it didn’t cost a lot of money either. One of my favourite apps.
I would definitely recommend you to use a small specially developed brass tuner that you attach directly to your instrument with the clips(on the trumpet just in front of your eyes on the bell). Look for the KORG AW-LT100T. Small and discrete. Choose various frequency and fast or slower response.