Android: Extract the main content CucumberInstrumentation

To simplify custom instrumentation classes inheriting from other
instrumentation base classes, basically all content of
CucumberInstrumentation is extracted to the new helper class
CucumberInstrumentationHelper.
Rename CucumberInstrumentationHelper -> CucumberInstrumentationCore
Merge pull request #667 from brasmusson/android-espresso-support

Android: Prepare for Espresso support