[flexiglass] Adds authmethod flow. am: 1cf0f476a3 am: 16d28829f6

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

Change-Id: If7314b00383435fc0b894ce0836ec3948487838a
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>