libfsmgr: SplitFiemap: helper to write a list of split files.

A helper function to write a file with a list of filepaths is extracted
from SplieFiemap::Create().

This is to reuse the helper in a follow-up change.

Bug: 402256229
Test: fiemap_writer_test
Change-Id: I72fa62f47f7a4634166b59bb562113fc6c6a1709
1 file changed