Fix output bugs in FnApiDoFnRunner @OnTimer and @OnWindowExpiration

[kennknowles]

The three argument providers each carry their own near-identical copy of the output-receiver plumbing, and the @OnTimer and @OnWindowExpiration copies had drifted from the working @ProcessElement one:

• OnTimerContext.outputWindowedValue(tag, ...) had an empty body, so tagged windowed output from @OnTimer was silently dropped.
• OnTimerContext's tagged receivers passed no tag to outputWindowedValue, so output to a side tag went to the main output.
• The @OnTimer row receivers built from WindowedValues.builder( currentElement) or called withValue() on a fresh builder. Neither works during timer processing: currentElement is only set while processing an element, and withValue() reads timestamp/window/pane off the builder it is called on. Both now seed from currentTimer, matching the @OnWindowExpiration equivalents.
• OnWindowExpirationContext read currentElement.getValueKind() while currentElement was null. Window expiration emits new records, so this is ValueKind.INSERT.
• OnWindowExpirationContext.timeDomain() returned currentTimeDomain, which processOnWindowExpiration never sets. TimeDomain is not an allowed @OnWindowExpiration parameter, so drop the override and let BaseArgumentProvider reject it.
• outputWindowedValue(tag, ...) skipped the unknown-tag check its siblings perform, so a bad tag produced an NPE.
• processTimer's finally did not clear causedByDrain, unlike processOnWindowExpiration. Not observable today since every entry point sets it before invoking user code, but the asymmetry is a trap.

The two inner Context classes are renamed to TimerContext and WindowExpirationContext so the fields holding them can drop their raw types; the enclosing providers are generic in K, and errorprone's SameNameButDifferent rejects the bare Context name.

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