Refactor crostini notification time remaining message for reuse

Bug: 912638
Change-Id: I3f99f31e50b14ea51d2b871872bf447911d426eb
Reviewed-on: https://chromium-review.googlesource.com/c/1476872
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Reviewed-by: Timothy Loh <timloh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633037}
4 files changed