[Tabs] Create TabsKeyManager to reduce duplication between tabs and stepper

[g1shin]

md-tab and md-stepper both have tab roles that follow the same keyboard interaction pattern.

Creating TabsKeyManager will reduce duplication between tabs and stepper components.

[crisbeto]

This one shouldn't be necessary anymore since the ListKeyManager supports horizontal directions.

[crisbeto]

Closing since ListKeyManager has had horizontal support for a long time now.

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