Switch to HexEncoding from ByteStringUtils

Migration in preparation for deletion of ByteStringUtils.

This change also fixes some import issues necessary for upload.

Bug: 124232146
Test: build only
Exempt-From-Owner-Approval: Already approved internally
Merged-In: I6ee12e9914503a5b628ae6da82a396aa71dd972c
Change-Id: I6ee12e9914503a5b628ae6da82a396aa71dd972c