Switch over from libbrillo flags to gflags am: 77cb1c6b3b am: 9e81651a6b am: f795272f34 am: 847190d961

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

Change-Id: I0840816b1180618c35263bf6208b906b8847fd27
Signed-off-by: Automerger Merge Worker <[email protected]>