[flexiglass] Drive window view visibility.

When the internal state of our scene system is updated, the visibility
of the window view that contains our scene graph is driven by the new
startable.

Bug: 290656769
Test: added unit tests
Test: manually verified that, when the feature flag is enabled, the
visibility calls coming from legacy code into
WindowRootView#setVisibility are properly ignored and the visibility of
the root view is driven by the new system instead.
Test: manually verified no harm done. When the feature flag is disabled,
the visibility of the window root view is still updated by the legacy
code.

Change-Id: I20dccfdc7089dda0732b56c384d139e79a4c2f96