Fix stalled background thread bug

This change forces the smart reply action icon load to happen on a new thread (using a thread pool to avoid bursts of thread allocations in nominal reinflate situations) and has a 500ms timeout.

Fixes: 283789325
Test: Running Proof of Concept app, blocking, and posting lots of notifications
Change-Id: Ided53dfd0c696fb5895d8b55806cb0219d8a0624