Deflake NotificationEntryManagerInflationTest

Deflake NotificationEntryManagerInflationTest by moving back
to the countdown latch approach and providing our own executor
to AsyncInflationTask.

It was infeasible to depend on the exact number of main thread messages
as other main thread messages from other tests could interfere. Thus.
we move back to the countdown latch for when NotificationEntryManager
finishes inflation. This makes us dependent on the listener API for
determining when inflation is finished, but it's unlikely we can do
better unless we can inject an executor into RowInflaterTask.

In addition, using AsyncTask's SERIAL_EXECUTOR on hwasan builds seemed
to be much slower than normal. This lead to other tests' AysncTask
work delaying the work in this test and leading to the timeout
happening before inflation finished. By providing a test executor
instead and synchronously controlling its execution, we avoid this
issue.

Bug: 150618180
Test: atest --iterations 100 NotificationEntryManagerInflationTest
Test: atest SystemUITests
Test: A forrest run coming soon!
Change-Id: I87c6e2d216c8f26aaf340d311f618c9dccaba8af