floss: fix log.h dependency of init_flag for ChromeOS

Fix the log.h missing dependency of init_flag for ChromeOS by adding
init_flags.h when building for Floss. In addition, update BUILD.gn
files to include init_flags_bridge_header wherever log utilities are
used.

BUG: 232098615
Tag: #floss
Test: emerge-${BOARD} floss
Test: ./build.py
Change-Id: Ieac199eece21444d79003382d2e33fe337162b06