Fix selectors for MdCardXlImage and MdCardAvatar

[mattlgy]

I believe the selectors for the MdCardXLImage and MdCardAvatar directives should to attribute selectors like the rest of the card image directives.

Fixes #3131

[kara]

LGTM

[stefanaerts]

in previous material versions it was this
[md-card-avatar] {
height: $md-card-header-size;
width: $md-card-header-size;
border-radius: 50%;
}
now its this
.mat-card-avatar {
height: $md-card-header-size;
width: $md-card-header-size;
border-radius: 50%;
}

so if you use class="mat-card-avatar" instead of md-card-avatar
it will work

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