Demux thread reads data after filter start

Demux thread should read socket data only when there are active
filters reading data. When a filter is started, the reading
thread on the demux is notified of the active filter, and it
starts reading data. When the last filter is stoped, the thread
is notified and it stops reading data.

Bug: 288170590
Test: manual
Change-Id: Idd380bc0d86c445ce9faef8e445d636bbe4e91fc