Add new atoms to track user journeys, such as user switches.

Define new atoms to better log and understand various user journeys,
such as user switches, user starts, and user creation.

The UserLifecycleJourneyReported atom defines a user's journey and
holds a user's information such as their user id, user type, and flags
associated with the user. This atom includes a session-id which is used
to link to the UserLifecycleEventOccurred atom which keeps track of
lifycycle events that occur throughout a user's journey. As more user
journeys are defined, there could be more Events added in the future.

Note: the JourneyReported atom can currently be logged more than once
per user journey since there is no record of the user's ongoing journey.
This will be updated in a future CL.

Bug: 146505521
Bug: 150788910
Test: statsd_testdrive 264 265
Change-Id: Iff3847be64d718fb2ec17e58c33d47f7fa4b627a