Bottom sheet needs focus management

[marcysutton]

To be accessible, when a Bottom Sheet component is opened, focus should be sent to its first interactive control ("Share" in the Show As List option). This will vastly improve the experience for keyboard and screen reader users. Currently, focus is blurred from the triggering button; this causes focus to be lost, a major accessibility issue.

[marcysutton]

Also, the items in the bottom sheet demo are simply <md-item> components and are not accessible even with ngAria in use. Moving the ng-click to a button element immediately inside of <md-item> solves the issue.

[marcysutton]

@rschmukler I may need your help with this one, and we need some test coverage for it.

[ThomasBurleson]

Recent changes to $mdBottomSheet must be resolved before an autoFocus-on-show feature.

[ajoslin]

Working on this.