Publish chunk size in utility.h

The chunk size of the snapshot should be kept consistent among different
files.
Move it to libsnapshot/utility.h to improve its visibility.

Change-Id: I1cae6530a07c88f3a0091ca7d8bb2eb590a6710f
Bug: 140835698
Test: m
Signed-off-by: Alessio Balsini <balsini@google.com>