Inflate notification content on a dedicated thread

We're worried that incoming call notifications might be getting delayed
by other background activity, so we're moving notification content
inflation to a dedicated background thread.

Bug: 308967184
Flag: ACONFIG dedicated_notification_inflation_thread DEVELOPMENT
Test: existing notification tests
Change-Id: I4f6331bf3ae21ae5871a6f16f6235af2412950c5
4 files changed