Define priority for video threads

Video threads have soft realtime requirements that
must be met in order to maintain reliable frame delivery
even when the system is under high load. A new priority
has been defined that can be used by video threads to
enable them to be scheduled appropriately relative
to other system threads. This CL updates the media codec
looper to use the new priority.

Tests: GTS media tests, Manual verification with
Play Movies, Netflix, com.google.vr.sdk.samples.videoplayer

Change-Id: Ibfee09bb6a5359ab96ec2184775c486e4bd77871
related-to-bug:63898080