Add source change event in V4L2 Jpeg decoder accelerator

Some platforms will generate different output subsampling formats with
different input subsampling formats. Therefore, we enqueue input buffer
first and then use S_FMT to get proper pixelformat and resolution of
output buffer.

Without pixelformat change event, we cannot know which buffer has format
change when there are multiple buffers queued. When source change event
is triggered, driver has stopped decoding. Client should dequeue event,
dequeue all output buffers, and stop CAPTURE queue. Then the client
should call S_FMT to set output pixelformat. If driver doesn't support
the assigned pixelformat, driver should overwrite a supported format.
Client has to call STREAMON on CAPTURE queue again to continue decoding.

This commit also adds the support to convert the output format to I420
if the driver does not support I420.

BUG=426383
TEST=apprtc without loopback, pass JDA unittests.

Review URL: https://codereview.chromium.org/1279863006

Cr-Commit-Position: refs/heads/master@{#355507}

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