feat(material/radio): allow focus origin to be optional input in focus method

[jesscxu]

WHAT: For Angular Material components that have a focus() method, allow for the origin param to be optional and remove the default origin value.

WHY: For cases where the focus() method is called and the origin is already defined, we don’t want to override the origin using focusVia to always be some default value. In many cases, we want to leave the origin unchanged, but if there are cases that need the origin to be updated, allow for this with an optional origin param.

[jelbourn]

Would this change also need to apply to the MDC-based radio button?

[jesscxu]

@jelbourn It should already be applied because the focus() method I am updating is in the class _MatRadioButtonBase and MDC-based radio button extends this class. This change should apply to both MDC and non-MDC radios.

[mmalerba]

LGTM

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