Move global analysis options file to the repo root (#3750)

* Move global analysis options file to the repo root

Also restore the analysis options file for stocks example, since
the global options trigger warnings on generated intl files.

* Remove obsolete analysis options file for Material Gallery example

examples/material_gallery/.analysis_options → examples/stocks/.analysis_options

 analyzer:
   exclude:
-    - 'ios/.generated/**'
+    - 'lib/i18n/stock_messages_*.dart'