Add exclusive attribute to suggestion category

Add exlusive attribute to suggestion category XML, which when true,
will prevent all other categories from being shown in suggestions.

If there are multiple suggestions in a single exclusive category, all
of them will be shown.

If there are mutliple exclusive categories with enabled suggestions,
only the first will be shown.

Test: cd tests/robotests && mma
Bug: 35668836
Change-Id: I8b95cdcd021a2d67d460259b4a44ca37290ffd48