fix(ui5-list): improved acc description when groups are available

[plamenivanov91]

Improved ACC description for list when we have groups.

Depending on the list role, the description differs.

[yanaminkova]

The DOM structure of the ui5-list with group header should be changed as well, as shown in the BLI:
Grouped List with focusable group headers:

<h1 id="title">My Title</h1>
<ul role="list" aria-labelledby="title" >
	<li role="listitem"aria-labelledby="4_group1_label" tabindex="0">
		<span id="4_group1_label">Group 1</span>
		<ul role="list" aria-labelledby="4_group1_label" aria-owns="4_item1 4_item2"></ul>
	</li>

	<li id="4_item1" role="listitem" </li>
	<li id="4_item2" role="listitem" >Pears</li>

	<li id="4_group2" role="listitem"aria-labelledby="4_group2_label">
		<span id="4_group2_label">Group 2</span>
		<ul role="list" aria-labelledby="4_group2_label" aria-owns="4_item3 4_item4 4_item5"></ul>
	</li>

	<li id="4_item3" role="listitem"></li>
	<li id="4_item4" role="listitem"> </li>

</ul>

Grouped Listbox with non-focusable group headers

<ul role="listbox" aria-labelledby="title" aria-description="contains 2 sub groups">
	<li id="5_group1" role="group"aria-labelledby="5_group1_label" aria-owns="5_item1 5_item2" class="Header">
		<span id="5_group1_label">Group 1</span>
	</li>

	<li id="5_item1" role="option" </li>
	<li id="5_item2" role="option" tabindex="0" aria-posinset="2" aria-setsize="5" class="Item">Pears</li>
	<li id="5_group2" role="group" aria-labelledby="5_group2_label">
		<span id="5_group2_label">Group 2</span>
	</li>

	<li id="5_item3" role="option">Apples</li>
	<li id="5_item4" role="option"></li>
</ul>

For reference have a look at:
https://sapui5untested.int.sap.eu2.hana.ondemand.com/#/entity/sap.m.List/sample/sap.m.sample.ListGrouping

where the change is already made