Support for CHROMIUM context for embedded Chromium.

Enables switching between NATIVE_APP and CHROMIUM contexts. Works for both backends: UIAutomator and Selendroid. It's intended to work for any Chromium
embedding app under the following conditions:
- user has to make sure the devtools socket is open
- user may need to provide a webdriver working with his app (probably deriving from chromedriver)
- user needs to use some additional capabilities: 'android-device-socket', 'chromedriver-executable' (optional), 'special-chromedriver-session-args' (optional)
Merge pull request #2435 from emakiej/support_chromium_context

Support for CHROMIUM context for embedded Chromium.