liblp: Expand the metadata header for future use.

A few times we have wanted to stash small bits of information in the
super header, but we haven't had any bits to do so. This patch addresses
future needs in two ways:

  1. A "flags" field has been added for miscellanious bits that do not
     need a version bump.
  2. The header struct has been padded to 256 bytes to allow for future
     expansion without complicating the struct-parsing code.

This is the first time we've materially changed the format, so this
patch needs some extra explanation.

In all the places we rely on sizeof(LpMetadataHeader), we now need to
use the |header_size| field instead. To make newer versions of liblp
compatible with older headers, we read the minimum required header size
and fill in the extra bytes as needed. To make the validation and
reading logic more clear, it is now combined into a single function,
ReadMetdataHeader.

MetadataBuilder will still emit 1.0-compatible headers, to avoid
changing the on-disk format of existing devices. The new header will
only be emitted as-needed.

Bug: 134949511
Test: liblp_test gtest
      retrofit DAP device boots
      launch DAP device boots

Change-Id: I6221123768ff0057a73967ecb2ff9b006c17af88