SDK / exoplayer

- This is to make sure we know about all the tracks before initializing
the SamplePipelines. This allows to set the muxer and the fallback
listener track count before the SamplePipelines are built.
- As a result, the test files had to be updated because the order in
which the tracks are written has changed.
- The ImageAssetLoader also had to be updated to call onOutputFormat
repeatedly until it returns a non-null SampleConsumer.
- Also fix the trackCount sent to the muxer and fallback listener. The
correct track count can be computed now that we know about all the
tracks before building the SamplePipelines.

PiperOrigin-RevId: 514426123