Audio encoding is usually mono-threaded and doesn't require
as much CPU as video encoding. Limit to 4 threads since more
would not be useful and might introduce latency and decrease performance.

Video encoding takes significantly more resources, however for live encoding,
using more than 16 threads might introduce unreasonable latency.
Since new customer CPUs (AMD Ryzen and Intel Core i7/i9) now have
more than 16 logical cores, setting a higher bound of 16 threads.

Change-Id: I5dc2d3f51019c563b8cec3fbddffa13cbafdb9a5