Switch to HexEncoding from ByteStringUtils

Migration in preparation for deletion of ByteStringUtils.

This commit switches bluetooth unit test code from
android.util.ByteStringUtils to libcore.util.HexEncoding.
Neither are public APIs but ByteStringUtils is being deleted.

Bug: 124232146
Test: build only
Change-Id: I6a7eaf75ed05a596f9041a123f50814c0877bcbb