Replicated backend in a new espressoBackend package.

 Renamed InstrumationBackend class to RobotiumInstrumentationBackend and EspressoInstrumentationBackend respectively.

For now we will have 2 mirror packages with all the classes. When the Espresso implementation finishes we can consider extracting common resources to a super package for classes with no relation at all with the instrumentation used, like most utility classes and a lot of the query stuff.