libsparse: Add method to create sparse file from buffer

Refactor elements of sparse file parsing that depend on
an fd into SparseFileSource class, then create implementations
using both fd and buffer. Add sparse_file_read_buf which
reads the given buffer into a sparse file cookie without
copying.

Test: flash system with sparse images
Bug: 78793464
Change-Id: Ice6c8e1ff075d6867e070f80fcf5aa4f530a1b95