fix(ui5-avatar): fix XS size

[ilhan007]

With the introduce of the AvatarGroup the XS size of the Avatar has been broken. Due to the following css the XS size appeared 3rem X 3rem, instead of 2rem X 2rem (as it is by spec). The "XS" styles define min-height and min-width, but the default styles make it 3rem by 3rem.

:host {
	height: 3rem;
	width: 3rem;
}
:host([_size="XS"]),
:host([size="XS"]) {
	min-height: 2rem;
	min-width: 2rem;
	font-size: .75rem;
}

[nnaydenow]

Minor: we could add sample with avatar-size="S" just for test purpose.

[ilhan007]

Minor: we could add sample with avatar-size="S" just for test purpose.

added more samples

[dobrinyonkov]

Looks good to me.