build: format mdc-slider build file imports (#16919)
build: format mdc-slider build file imports (#16919)

When the mdc-slider change landed, the PR was not rebased on top
of the latest changes in `master`. Meaning that the buildifier alphabetical
sorting rule was not enabled and the build file now fails the sort rule in
`master`.
andrewseguin/material2,josephperrott/material2,mmalerba/material2,mmalerba/material2,Jefiozie/material2,mmalerba/material2,crisbeto/material2,crisbeto/material2,mmalerba/material2,DevVersion/material2,DevVersion/material2,andrewseguin/material2,jelbourn/material2,andrewseguin/material2,jelbourn/material2,josephperrott/material2,andrewseguin/material2,angular/components,Jefiozie/material2,DevVersion/material2,Jefiozie/material2,angular/components,jelbourn/material2,jelbourn/material2,josephperrott/material2,crisbeto/material2,jelbourn/material2,DevVersion/material2,andrewseguin/material2,josephperrott/material2,mmalerba/material2,Jefiozie/material2,DevVersion/material2,crisbeto/material2,josephperrott/material2,angular/components,Jefiozie/material2,angular/components