aggregator: Actually handle NEED_DATA return from update_src_caps()

The documentation says that this allows the subclass to signal that it
needs more data before it can decide on caps, so let's actually
implement it that way.
22 jobs for backport-mr-249-into-1.16-aggregator-update-src-caps-need-more-data in 69 minutes and 14 seconds (queued for 7 seconds)
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  Preparation
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gst indent

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manifest

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  Build
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build cerbero android universal

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gst-ios-12.2
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gst-macos-10.14
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  Test
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integration testsuites fedora

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  Integrate
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android universal examples

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