[Espresso-Flutter] Add API and test cases for supporting multiple FlutterViews

1) Add API for selecting a FlutterView when multiple FlutterView exist on screen;
2) Also add a test case with two FlutterViews on screen and let them talk with each other.

Start in the Espresso-Flutter API and its existing tests, then trace how a single FlutterView is currently selected. Add coverage for an app displaying two FlutterViews and verify that each can be selected and communicate independently. Done means the public API supports choosing either view and the two-view test passes.

Written by the indexing model from the issue text.