[flutter_driver] Fix flutter driver to enable integ tests

- flutter driver was connecting to moduleContext but never using it
- moved TestRunner.fidl connection to the test file since it needs
  to be exposed to test author for future use.
- Changed IntentHandler and Intent to allow null action intents.

This makes _extension_wrapper use async bindings and the new sdk.

Test:
fx shell /pkgfs/packages/topaz_modular_integration_tests/0/bin/run_topaz_modular_integration_tests.sh slider_mod_tests

DX-625 #done

Change-Id: Id7b0e69075cbd0261e07996f438ed130a833159a
7 files changed