Set an accessibility identifier on the default back button.
Set an accessibility identifier on the default back button.

Summary:
The back button created by the AppBar didn't have an accessibility identifier.
The accessibility identifier is convenient in UI tests.

Test Plan: Check that App Bar back buttons have an accessibility identifier.

Reviewers: featherless, randallli, O1 Material components iOS

Reviewed By: randallli, O1 Material components iOS

Subscribers: randallli

Tags: #material_components_ios

Differential Revision: http://codereview.cc/D935

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