Use ADB to take screenshots of android devices

[r1sim]

This PR implements ADB screenshots for android devices as discussed in #175

Nexus 6P Before	Nexus 6P After

Main Changes:

• Added adb_path & adb_device_id to config.screenshotsEnv
• capture_screen.dart now checks if the new variables in config.screenshotsEnv are set and if the device_type is android. If that is the case it will attempt to take screenshots using adb. It will use FlutterDriver to take the screenshots if the adb method fails for whatever reason.
• The new screenshot method for android uses adb to take screenshots and to setup the navbar
• The unprocessed screenshots now have the extension .adb.png or .driver.png so the image processor knows how to process them. (Add Navbar or not)
• I upgraded the SDK version of the example app to a more recent version.

This works well with my setup. If it works as well for everyone else we could maybe think about dropping FlutterDriver support for android devices. This would make it much easier to add new devices because we would no longer need to add a statusbar and navbar for every resolution. The only downside of this would be that demo mode requires android 6.0.