Move quiet mode operations to a separate thread

The setQuietModeEnabled method is often called asynchronously on a
background thread. This has been a reason for noticeable delays caused
when the system server background thread is busy. With this change, we
are moving the operation to a separte thread using a ThreadPoolExecutor
of size 1.

Test: Tested locally on device by adding logs and capturing pertto
traces
Bug: 303201022

Change-Id: I71faf0d11115e5fa4828632c28778d3add9e62f2