This change creates the scenic session with request to Focuser. The
focuser is used to request focus to a view, given it's ViewRef. The call to
requestFocus is made from PlatformView messages channel, which will
be used from Flutter side by ChildView.
Adds a PlatformView unittest for the requestFocus method.
Co-authored-by: Sanjay Chouksey <sanjayc@google.com>