Fix accessible aria activedescendant attribute

Remove aria-activedescendant attribute after the dropdown is closed,
to make sure the aria properties is vaild for aduit-tool
[https://code.google.com/p/accessibility-developer-tools/wiki/AuditRules#AX_ARIA_04:_ARIA_state_and_property_values_must_be_valid]
Merge pull request #2875 from Casecommons/master

Fix accessible aria activedescendant attribute