Add type to device context

[wkunert]

What does this PR do?
This PR adds the type to the device context (Customer.io for example requires the device type to be set [alongside the device token] in order to recognize devices). The Segment iOS SDK is already setting the type.

How should this be manually tested?
Check in Segment’s event debugger if new events contain the type key within their context/device dictionary.

Any background context you want to provide?
We are developing in React Native and are using Firebase Cloud Messaging (via React Native Firebase). We want to use Customer.io to send out push notifications, so we are working on a pull request for the Segment React Native SDK to add functionality which is required to make this happen. Before we can continue our work we need a way to set the device type in Android – hence this pull request.

What are the relevant tickets?
segmentio/analytics-react-native#38

Questions:

• Does the docs need an update?
  No.
• Are there any security concerns?
  No.
• Do we need to update engineering / success?
  No.