Switch over from libbrillo flags to gflags am: 77cb1c6b3b am: 9e81651a6b am: f795272f34

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

Change-Id: I0598c393549dc15593c1baf1f6e38e06149f9dc9
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>