[AppBar] Respect the navigation bar's layout direction
[AppBar] Respect the navigation bar's layout direction

Summary:
The back button now faces to the right direction based on the
navigation bar's layout direction.

Closes https://github.com/google/material-components-ios/issues/564.

Test Plan:
Open MDCCatalog with Pseudo RTL or an RTL language.
Go to a component page.
Check that the back button points to the right.

Reviewers: iangordon, O1 Material components iOS, featherless

Reviewed By: O1 Material components iOS, featherless

Tags: #material_components_ios

Differential Revision: http://codereview.cc/D942

Eccelor/material-components-ios,chriscox/material-components-ios,randallli/material-components-ios,chriscox/material-components-ios,willlarche/material-components-ios,willlarche/material-components-ios,samnm/material-components-ios,brianjmoore/material-components-ios,randallli/material-components-ios,randallli/material-components-ios,willlarche/material-components-ios,Eccelor/material-components-ios,sigito/material-components-ios,material-components/material-components-ios,material-components/material-components-ios,sigito/material-components-ios,samnm/material-components-ios,chriscox/material-components-ios,willlarche/material-components-ios,brianjmoore/material-components-ios,brianjmoore/material-components-ios,chriscox/material-components-ios,samnm/material-components-ios,Eccelor/material-components-ios,samnm/material-components-ios,sigito/material-components-ios,brianjmoore/material-components-ios,randallli/material-components-ios,material-components/material-components-ios,material-components/material-components-ios,sigito/material-components-ios,Eccelor/material-components-ios,randallli/material-components-ios,brianjmoore/material-components-ios,Eccelor/material-components-ios,willlarche/material-components-ios,randallli/material-components-ios,material-components/material-components-ios,samnm/material-components-ios,chriscox/material-components-ios,sigito/material-components-ios