Add time info to travel behavior activity

[cagryInside]

Timestamps are added to transition events. For each transition event we collect the following timestamps:
(A) -> TransitEvent.elapsedRealtimeNanos() ,
(B) -> SystemClock.elapsedRealtimeNanos(),
(C) -> System.currentTimeMillis(), and
(D) -> (C) - (A) = number of nanoseconds in the past when the event happened.

The data looks like the following in Firebase:

[barbeau]

@cagryInside Thanks for implementing this! So, I made a mistake in the original Google drawing describing this, which I've corrected here:
https://docs.google.com/drawings/d/1AHvO62fClfBOfXmfUGWUltg8hnjkXitohq4FYq1quho/edit?usp=sharing

Here's the correct version:

(A) -> event.elapsedRealtimeNanos() ,
(B) -> SystemClock.elapsedRealtimeNanos(),
(C) -> System.currentTimeMillis(), and
(D) -> (B) - (A) = number of nanoseconds in the past when the event happened.
(E) -> (C) - (D) = Clock time (milliseconds past epoch) when transition happened

I've changed the implementation to match this (including the new field "(E)" in bc27281, but could you please review to confirm I got it right?

I also changed the field name from "transitEventElapsed..." to just "eventElapsed", as it's shorter and a better description of the data (again, my fault from not correctly the text in the diagram).

[barbeau]

Sorry, I just rebased it to get rid of other unrelated changes, new commit of mine is f2377e3