[java] Fp16 fix for android/react native (#16832)
### Description
This PR splits out the FP16 conversions into a separate package we can
override in the android build with a version which works on old versions
of Android.
I'm not sure the android build system changes are correct as I haven't
got an android build environment configured on my workstation.
@YUNQIUGUO if the CI build fails we should follow up offline to get my
environment configured so I can iterate on it.
### Motivation and Context
Fixes the CI failure after #16703.
