liblp: Clean up public headers.

This changes reader.h and writer.h to be private includes. A new liblp.h
header now contains the public API surface of those two files, as well
as some miscellanious functions previously in metadata_format.h.

Bug: 79173901
Test: N/A
Change-Id: I40c5dda0c8e5765f8bccfd5c17b4c800b41be77b