MediaCodecTrackRenderer: Avoid excessive garbage generation

Looking up a long in a HashSet<Long> auto boxes the long and leaves
it for the GC. As decodeOnly is relatively infrequent it's much
better to do a simple linear search in a List<Long>. That way
we can avoid boxing every incoming time stamp value. In the general
case this will be linear searching in an empty list, a very fast
operation.

Signed-off-by: Jonas Larsson <jonas@hallerud.se>