Package fuzz targets into zipfiles, then coalesce.

Instead of writing all fuzz target files into a huge zipfile (by number
of entries), we instead write each fuzz target and its files to a
zipfile, then coalesce them into a big zipfile.

This should simplify the repackager significantly, as no unzipping and
repackaging should be needed (just renaming).

This also solves b/144418402, where we are very close to hitting the
POSIX args limit on the soong_zip command for the device fuzz package,
due to the line length generated by all the entries in the huge zipfile.

Fixes: 144418402
Test: make fuzz
Change-Id: I89797e1bea2691ff4a33a227306f2f9083d8af6e