[TG Sync] Move out JNI conversion utility to public

This CL moves out the JNI conversion utility for saved tab groups
to public subdirectory. This is already being used by messaging
and can be reused by more components in future. Hence this should
no longer be internal code and can be moved to public.

Bug: 369624428
Change-Id: Ib6a637edfe5e1cb48e72b728804a633cb308259b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5907183
Commit-Queue: Shakti Sahu <shaktisahu@chromium.org>
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1363680}