unicodesymbols: Use \r{A} instead of \AA for "Latin letter A with ring" and "Ångström...

unicodesymbols: Use \r{A} instead of \AA for "Latin letter A with ring" and "Ångström sign".

Xe/LuaTeX convert \AA to the deprecated character u212B (which is missing
in the default LatinModern font) instead of the recommended u00C5.

Also fix some of the "missing character" errors in Math.lyx if compiled with
Xe/LuaTeX which were caused by the replacement of \AA with literal u212B characters
in math-insets due to the old definitions in unicodesymbols.

Update the minimal example for failures of Math.lyx with system fonts.