Add device_id as an automatic context value for Feature Flags

[msiebert]

When creating the feature flags context, we need to use anonymous ID as a context property with the name device_id. This property should be added at the same as we add distinct_id.

Additionally update test cases that verify the shape of context to check for device_id being present and having the right value. These can likely by found by searching for places where distinct_id is validated.

An example of implementing this in the Android SDK can be found in this PR.