liblp: Refactor ParseMetadata to read from non-descriptors.

This is in preparation for "fastboot flash super", where we want to
verify the validity of a super image before writing it. To do so, we
need to parse the image from the download buffer, and it is useful to do
this from memory rather than a file descriptor.

Bug: 78793464
Test: liblp_test gtest
Change-Id: I7fb1ef4fdf2e8f1d93aea38f75626157fcb4bfc1