Support FiemapWriters that extend across multiple files.

This introduces a new SplitFiemap class that will divide an allocation
request across multiple FiemapWriters. This is primarily useful on
filesystems that have onerous restrictions on maximum file sizes. Vfat,
for example, supports a maximum of 4GiB, which is too small to satisfy
larger userdata size requests.

Bug: 126230649
Test: fiemap_writer_test gtest
Change-Id: I3c95d341e4e94e0c44bbf0e8553c34ccfdcd155b