liblp: Export WriteToImageFile given a file descriptor.

This exports an internal variant of WriteToImageFile, since lpadd needs
to call it on an open descriptor. While we're here, switch "int" to
"borrowed_fd".

Bug: 144468779
Test: m lpadd
Change-Id: I2dd35b8a872f8a72a704230d0382398b8982cb0b