Change focus-trap control attributes to camelCase

[jelbourn]

cdkFocusTrap currently uses the attributes cdk-focus-initial, cdk-focus-start, and cdk-focus-end. We should change these to camelCase for consistency (even though they aren't directive selectors).

[jelbourn]

cc @mmalerba

[willshowell]

I think this can be closed.

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