Revise python streaming mobile gaming examples.

I spent some time trying to run streaming examples [1] on Direct runner and ran into the following issues:

• Examples use deprecated code as per warnings 
  • DeprecationWarning: options is deprecated since First stable release.. References to <pipeline>.options will not be supported
    project = pipeline.options.view_as(GoogleCloudOptions).project
    /tmp/release_testing/r2.4.0_env/local/lib/python2.7/site-packages/apache_beam/runners/direct/direct_runner.py:337: DeprecationWarning: options is deprecated since First stable release.. References to <pipeline>.options will not be supported
    pipeline.replace_all(_get_transform_overrides(pipeline.options))
• Path to Injector has a typo (java8 instead of java)
• I was not able to use the injector to produce an output on a pubsub topic, but perhaps I missed some instructions. Injector would exit after:
  • Warning! You are not using service account credentials to authenticate.
    You need to use service account credentials for this example,
    since user-level credentials do not have enough pubsub quota,
    and so you will run out of PubSub quota very quickly.
  • Note that I set an environment variable GOOGLE_APPLICATION_CREDENTIALS and also tried passing -DGOOGLE_APPLICATION_CREDENTIALS=/path/to/credential.json.
• Examples say they are not applicable for Dataflow runner, is it still the case?
• Running injector requires mvn, which is being replaced with gradle and also requires familiarity with java toolchain, even though the example is for Python SDK.

[1] https://github.com/apache/beam/blob/master/sdks/python/apache_beam/examples/complete/game/game_stats.py.