accessibility: improved conversations filters
accessibility: improved conversations filters

Fixes CNVS-14505

Test plan:
- Visit the inbox
- Using a screen reader, verify that the currently selected item is read as
  the label of the button to open the message filter drop-downs
- Verify that screen reader users can navigateand select items from both lists
- Verify that the appearance remains unaltered.

Change-Id: I1e177ca0be0d6b0c0ad6094de477938dfd440868

shidao-fm/canvas-lms,Jyaasa/canvas-lms,gbleydon/canvas-lms,leftplusrightllc/canvas-lms,narigone/canvas,HotChalk/canvas-lms,fronteerio/canvas-lms,gbleydon/canvas-lms,sdeleon28/canvas-lms,kyroskoh/canvas-lms,eCNAP/CANVAS,shidao-fm/canvas-lms,grahamb/canvas-lms,antoniuslin/canvas-lms,HotChalk/canvas-lms,sfu/canvas-lms,eCNAP/CANVAS,whalejasmine/canvas-lms,juoni/canvas-lms,AndranikMarkosyan/lms,instructure/canvas-lms,redconfetti/canvas-lms,wye0220/shindig-canvas,redconfetti/canvas-lms,Rvor/canvas-lms,roxolan/canvas-lms,ShreniiNepal/shrenii_lmp,HotChalk/canvas-lms,narigone/canvas,skmezanul/canvas-lms,utkarshx/canvas-lms,fronteerio/canvas-lms,redconfetti/canvas-lms,samuels410/greatlakes-lms,jay3126/canvas-lms,fronteerio/canvas-lms,efrj/canvas-lms,fronteerio/canvas-lms,matematikk-mooc/canvas-lms,Jyaasa/canvas-lms,venturehive/canvas-lms,redconfetti/canvas-lms,juoni/canvas-lms,ShreniiNepal/shrenii_lmp,instructure/canvas-lms,samuels410/greatlakes-lms,jay3126/canvas-lms,samuels410/greatlakes-lms,sfu/canvas-lms,wrdsb/canvas-lms,efrj/canvas-lms,whalejasmine/canvas-lms,dgynn/canvas-lms,wye0220/shindig-canvas,instructure/canvas-lms,tgroshon/canvas-lms,kyroskoh/canvas-lms,utkarshx/canvas-lms,jay3126/canvas-lms,snehakachroo1/LSI_replica,kyroskoh/canvas-lms,sfu/canvas-lms,ShreniiNepal/shrenii_lmp,greyhwndz/canvas-lms,whalejasmine/canvas-lms,Jyaasa/canvas-lms,SwinburneOnline/canvas-lms,venturehive/canvas-lms,antoniuslin/canvas-lms,wrdsb/canvas-lms,snehakachroo1/LSI_replica,va7map/canvas-lms,sfu/canvas-lms,grahamb/canvas-lms,juoni/canvas-lms,tgroshon/canvas-lms,instructure/canvas-lms,SwinburneOnline/canvas-lms,va7map/canvas-lms,sdeleon28/canvas-lms,grahamb/canvas-lms,wrdsb/canvas-lms,djbender/canvas-lms,matematikk-mooc/canvas-lms,tgroshon/canvas-lms,roxolan/canvas-lms,va7map/canvas-lms,instructure/canvas-lms,shidao-fm/canvas-lms,efrj/canvas-lms,greyhwndz/canvas-lms,Rvor/canvas-lms,AndranikMarkosyan/lms,eCNAP/CANVAS,matematikk-mooc/canvas-lms,utkarshx/canvas-lms,ShreniiNepal/shrenii_lmp,instructure/canvas-lms,gbleydon/canvas-lms,va7map/canvas-lms,samuels410/greatlakes-lms,tgroshon/canvas-lms,dgynn/canvas-lms,grahamb/canvas-lms,matematikk-mooc/canvas-lms,efrj/canvas-lms,AndranikMarkosyan/lms,skmezanul/canvas-lms,SwinburneOnline/canvas-lms,sfu/canvas-lms,HotChalk/canvas-lms,leftplusrightllc/canvas-lms,whalejasmine/canvas-lms,SwinburneOnline/canvas-lms,sdeleon28/canvas-lms,Rvor/canvas-lms,narigone/canvas,wrdsb/canvas-lms,antoniuslin/canvas-lms,dgynn/canvas-lms,snehakachroo1/LSI_replica,utkarshx/canvas-lms,sdeleon28/canvas-lms,greyhwndz/canvas-lms,narigone/canvas,eCNAP/CANVAS,roxolan/canvas-lms,Jyaasa/canvas-lms,djbender/canvas-lms,leftplusrightllc/canvas-lms,venturehive/canvas-lms,kyroskoh/canvas-lms,leftplusrightllc/canvas-lms,Rvor/canvas-lms,instructure/canvas-lms,shidao-fm/canvas-lms,sfu/canvas-lms,juoni/canvas-lms,AndranikMarkosyan/lms,skmezanul/canvas-lms,snehakachroo1/LSI_replica,venturehive/canvas-lms,grahamb/canvas-lms,antoniuslin/canvas-lms,skmezanul/canvas-lms,jay3126/canvas-lms,wye0220/shindig-canvas,dgynn/canvas-lms,djbender/canvas-lms,greyhwndz/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,roxolan/canvas-lms