Fix OutputSampler's coder

[rohdesamuel]

Previously, the OutputSampler coder would always be the Value coder (from a windowed value). This would cause a mismatch between the encoding for the PCollection defined in the ProcessBundleDescriptor vs. what is being given to the OutputSampler. The fix is to always sample WindowedValues and give the OutputSampler the literal rehydrated coder.

Task #25064

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[rohdesamuel]

R: @pabloem

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[pabloem]

lgtm!