Switch over from libbrillo flags to gflags am: 77cb1c6b3b

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2518715

Change-Id: Ia8c958ed1041bc96516353b5eb8665df5b90dfc5
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>