Switch ByteStringUtils over to HexEncoding impl

libcore/ already contains the necessary logic and it has
tests.

This should be a functional no op and is a precursor to removing
ByteStringUtils. HexEncoding may be hoisted up to android.util
eventually as there are no users in libcore outside of tests.

Bug: 124232146
Test: build only
Change-Id: I13bcb03aac0b3f19c6e4b9ac97f3c13e5012dc9c