Modify BuildFileToJsonParser to use Gson's streaming parser.

Summary:
Instead of using our hand-tuned streaming parser, use the default one in Gson.
This also enables us to parse arbitrary JSON structures instead of only arrays
of arrays of primitives.

This changes `BuildFileToJsonParser` to take `isServerMode` as a constructor param
so that we are explicit. Despite what the comment on `BuildFileToJsonParser#isServerMode`
says, I don't believe that we can use "regular mode" going forward.
That is, because the output from `buck.py` includes an `__includes` entry,
we cannot read a single JSON object in isolation because there may be context
from the `__includes` entry that it needs.

Test Plan: Sandcastle builds.