[flexiglass] Adds authmethod flow. am: 1cf0f476a3

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/24296062

Change-Id: I127ec08b5eaccc35f5bf0d613939d5fa1b3f5678
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>