logger.h: Add definition for logger_entry_v4

Extends logger_entry_v3 to add an uid field and moves some fields to
unsigned (easier to have zero'd bit-field for future adjustments).

Still no version field, not crossing that bridge, historical reasons.

Bug: 25996918
Change-Id: I18b7291666c3a5b170d6771a012363a41f2c5847