Adds isMobileSegment property to VerticalSegmentModel

[oguzkocer]

This PR implements the FluxC side of wordpress-mobile/WordPress-Android#8999 by adding isMobileSegment property to VerticalSegmentModel. The endpoint returns this property as mobile, but I think this name makes it clear. I am open to suggestions for an even better name.

I've also re-enabled the limit check for the verticals connected test. This basically tests that the limit parameter for the /verticals endpoint is respected which has been recently fixed on the server-side.

To test:

1. Run all the connected tests in ReleaseStack_VerticalTest
2. Debug the ReleaseStack_VerticalTest.testFetchSegments by putting a breakpoint in ReleaseStack_VerticalTest.onSegmentsFetched and observe that every segment except for Online Store has its isMobileSegment set to true.

[kwonye]

Tests pass and I've verified Online Store is the only segment where isMobileSegment set to false.