Add mode transition markers for trip plans on map

[barbeau]

Summary:

Currently trip plans look like on the map:

I'd like to add more "sub-markers" that designate transitions between walk/bus, bus transfers, etc., and different modes when we support them.

See #671, which added start/end markers for trip plans - we should be able to similarly leverage the mFragment.getMapView().addMarker() method to do this.

Steps to reproduce:

1. Plan a trip
2. Tap on map view tab

Expected behavior:

I should be able to see where walking transitions to bus, and bus transfers

Observed behavior:

One line with start/end markers for the entire itinerary

[barbeau]

Here's how the markers look in OpenTripPlanner Android:

Here's how the markers look in Google Maps (left is using real-time location as origin, right is using a static location for origin):

[barbeau]

We could potentially enhance the current start/end markers too - I added the simplest possible ones for #671, which use the default map marker and a limited set of hue colors. I like the Google Map presentation of the blue/gray dot.

[barbeau]

This is being addressed as part of the bikeshare implementation (at least for bikeshare) via issue #402 and WIP PR #776.