24bit implies digital sound. Once the phone outputs the sound through its DAC (digital to analog converter) it is no longer 24bit. It is simply an analog signal. AUX inputs are analog. The "24bit" aspect of the audio never makes it to the headphone jack.
So yes, that's basically the only way to get sound out of a phone other than to send it via wireless means to something else that has a DAC.
Also, for what it's worth, no one would ever hear the difference between CD quality 16bit 44khz audio and anything higher in a car. With road noise, reflections, vibrations, etc., going beyond CD quality (and in most cases high bit-rate MP3) makes no difference in a listening environment as poor as a vehicle. Honestly, I wouldn't worry too much about it in any situation (in home, with headphones, etc.) unless you've got extremely expensive equipment and a golden ear.
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