liblog: support extended logger_entry headers

logger_entry has a hdr_size field meant for backwards compatibility,
however there are a few checks that expect it to be a specific size.
More problematically, it has a variable length array element, msg[0]
at the end, that would be incorrect if the header size was not kept
static.

This change fixes up those issues to facilitate future additions.

Bug: 119867234
Test: logging works with extended size headers, logging unit tests
Test: newly added unit tests with extended size header
Change-Id: I14b2b0855d219d11f374b53a5aa8e05cd6a65eef