Add missing duration target track metadata when adding track to muxer

[izzytwosheds]

On newer versions of Android MediaFormat produced by Encoder might not contain some useful values (such as duration) even if they were passed in. When such MediaFormat is added to MediaTarget it will produce a video with missing useful metadata, as reported in #222

We are fixing this by populating missing duration and language metadata from source MediaFormat in encoder output MediaFormat before adding a track in MediaTarget.

Assertions are added in unit tests for new logic.