Wait for IApplicationThread events via ping/pong.

To aid in writing faster-running tests, clients might be interested
in confirming that all pending IApplicationThread events have been
processed.

This change provides that mechanism by adding a ping/pong event
which goes through the same IApplicationThread serialization
ordering used for other events, and waiting for all running apps
to process that ping/pong before returning.

Bug: 269704322
Test: TH
Change-Id: Idbf265a734998cf49dd89bf939e17f900d6b1fdd