Redesign of Enabler app and major refactor of messages inside library

This release requires a VI that speaks v0.4.0 of the OpenXC message format.

The attached APK file for the Enabler is built directly from this code base with one addition - it includes a token for authenticating with Bugsnag so the OpenXC developers can track unexpected errors and crashes. If you do not wish to report crash information, please build the app from the open source codebase.

• BREAKING: Refactor RawMeasurement class into more useful message types in the
  openxc.messages package, all based on a VehicleMessage. This requires
  updates to all code that uses the VehicleManager.
  • Renamed addVehicleInterface -> setVehicleInterface, only allow 1 VI at
    a time.
  • getSourceSummaries and getSinkSummaries are removed in favor of
    getActiveVehicleInterface.
• BREAKING: Classes implemeting VehicleInterface must now implement an
  isConnected() method.
• BREAKING: Increase minimum Android API version from 8 to 9.
• BREAKING: Refactor VehicleManager's API for retrieving the active vehicle
  interface.
• BREAKING: Limit the VehicleManager (and Enabler) to only 1 active VI at at
  time, making the API and UI much more clear. Because of this change, the USB
  VI is no longer enabled by default - it must be explicitly selected.
• BREAKING: MockedLocationSink is renamed to VehicleLocationProvider, and it
  no longer implements VehicleDataSink. It is simply a VehicleMessage
  receiver and is instantiate explicitly in the VehicleManager.
• BREAKING: Message listeners will no longer receive the last known value of the
  message when they register. The complexity of maintaining this was not worth
  it since there is a synchronous API you can get to get the current value on
  startup anyway.
• Feature: Support sending diagnostic requests to a VI.
• Feature: Support receiving diagnostic responses from a VI.
• Feature: Add new KeyMatcher class to support matching multiple types of
  messages from VI with a single handler.
• Feature: Redesign Enabler UI for clarity and add new swipeable view panels
  • Dynamically list all received simple vehicle messages in Enabler
    instead of hard-coding a subset. Supports anything being sent by the VI with
    a name and value.
  • View a list of all received CAN messages, with timestamp, ID and data.
  • Send an arbitrary CAN message.
  • Send a diagnostic request.
  • View diagnostic responses.
• Improvement: Add full support for receiving and sending protobuf encoded
  messages to a VI (although the vi-firmware does not yet have full support
  for receiving commands encoded as protobufs, see
  vi-firmware#313.
• Improvement: Use Gson for JSON (de)serialization to greatly decrease code
  complexity in exchange for a small performance hit.
• Improvement: Use message injection instead of trace file playback in test
  suite for quicker, more reliable tests.
• Improvement: Only attempt to send commands to a VI if it claims to be
  connected.
• Improvement: Don't display Android's GPS location in Enabler to avoid battery
  drain.
• Improvement: Use more standard icons to show vehicle interface connectivity in
  Enabler, and include multiple resolutions for high-DPI displays.
• Improvement: Switch from Crashlytics to Bugsnag for crash reporting as it
  doesn't require an Eclipse plugin to get going.
• Improvement: If native GPS overwriting is disabled, fully disable the vehicle
  location provider instead of just leaving it on.
• Improvement: If native GPS injection is enabled, only register for GPS updates
  from the phone if connected to a vehicle interface. If not connected, don't
  keep GPS alive to avoid battery drain.
• Improvement: Greatly increase unit test coverage, from 8% to 57%.
• Improvement: Add a callback for VehicleDataSource instances registered with
  the VehicleManager to be notified when the pipeline is activated or
  deactivated.
• Fix: Warning displayed properly when remote vehicle service is not running.