libziparchive: Add ability to backup in ZipWriter

Based on the compressed size of a file entry,
the decision needs to be made to instead store the file
uncompressed. This adds support to ZipWriter to backup
its last file entry.

The file is now always truncated when the EOCD is written out,
to account for the case where a file entry is backed-up and the
resulting file written is much smaller, leaving garbage data at
the end of the file.

This change also includes a rename of FileInfo -> FileEntry.
This struct was private (now public), so it shouldn't affect any
clients.

Bug: 35461578
Test: make ziparchive-tests
Change-Id: I23dc584406274ab7b8ce62b3fbc3562ca4c2603e