MH2 | Implement pending writes thread

Spin up a background thread inside the hal proxy that is responsible for
writing events to the event fmq if a previous write failed on the normal
postEvents thread. Create several new unit tests that help test the new
functionality.

Bug: 136511617
Test: Unit tests passing.
Change-Id: Ic35c9736fc0402297ab50072c195f66c9feb887d