Add reference counter for Filter

Filters should start demux reading thread for IPTV when start()
is called and they should PAUSE the thread when stop() is called.
They should only be paused when all associated filters of the
demux are closed. This CL adds reference counters for filters to
track that.

Bug: 288170590
Test: manual
Change-Id: I087175a685a4559918ceaa511971b79e7fcb0a51