fix(ui5-calendar): select correct day in other month

[didip1000]

Issue:

Clicking a date from the prev/next month would select the wrong date. On mousedown, the calendar navigates to the prev/next month and re-renders before the click event fires. The click handler then reads the outdated data-sap-timestamp (whatever date that's currenlty at that grid position in the new month), and selects that instead of the originally clicked date.

Solution:

Use the timestamp already set by _onmousedown instead of re-reading from the DOM on mouse clicks, and restore focus after the re-render when an other-month date was clicked.

Fixes: #13760

[sap-ui5-webcomponents-release]

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