Android: Replace own pthread_barrier and iconv implementations by the one from Bionic

[JoergAtGithub]

In 2014 Android Bionic didn't had pthread_barrier and iconv implementations and therefore own implementations of them were added to the hidapi sources: 417b2bf#diff-516ed9d234e81614e6fa65cfc81a50b8b119e8a0769b2e66a1c89166bac437f7L44-R479

But since API level 24, which was released in 2016, Androids Bionic C standard library contains the pthread_barrier functions: https://android.googlesource.com/platform/bionic/+/master/docs/status.md

Since API level 28 (released in 2018), also iconv was added to Bionic.

[Youw]

For historical reasons: here is Android iconv implementation: https://android-review.googlesource.com/c/platform/bionic/+/464455/3/libc/bionic/iconv.cpp