PR #6214: Remove async promise function so that errors propagate correctly.
PR #6214: Remove async promise function so that errors propagate correctly.

Closes #6214

COPYBARA_INTEGRATE_REVIEW=https://github.com/polymerlabs/arcs/pull/6214 from shans:rearrange-ui-muxer 15b19a9aa745fdc157be029782ac26e6a335577b
PiperOrigin-RevId: 333459902

PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs