[engine] always force platform channel responses to schedule a task.

[jonahwilliams]

If we use runNowOrPostTask on platform channel responses, then we may not wake up the dart message loop. If nothing else wakes it up, then the embedder may hang on platform channel responses.

This fixes several google3 integration tests when run in merged thread mode.

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[johnmccutchan]

Should we make this conditional on merged thread mode?

[jonahwilliams]

Before merged thread mode, RunNowOrPost task would always force a task to be posted because it was not possible for platform channel responses to come from the UI thread - only from plugins on the platform thread. In merged thread mode, RunNowOrPostTask is immediately invoking the closure without scheduling a task - which means the dart event loop isn't worken up.

So all I'm doing here is removing the conditional behavior for merged thread mode which is always incorrect.

[jtmcdole]

Reason for revert: I believe this change caused flutter/flutter to break in an engine roll. It was one of two changes - the other being a webui change.

Tests broken:

module_test_ios: testDualCold

router test:

[2024-09-05 17:43:20.837343] [STDOUT] stderr: [ +135 ms] VMServiceFlutterDriver: Connected to Flutter application.
[2024-09-05 17:43:20.841927] [STDOUT] stdout: [   +4 ms] 00:00 �[32m+0�[0m: flutter run test --route sanity check flutter drive --route�[0m
[2024-09-05 17:43:20.875760] [STDOUT] stdout: [  +33 ms] 00:00 �[32m+0�[0m�[31m -1�[0m: flutter run test --route sanity check flutter drive --route �[1m�[31m[E]�[0m�[0m
[2024-09-05 17:43:20.877108] [STDOUT] stdout: [   +1 ms]   Expected: '/smuggle-it'
[2024-09-05 17:43:20.877135] [STDOUT] stdout: [        ]     Actual: '/'
[2024-09-05 17:43:20.877172] [STDOUT] stdout: [        ]      Which: is different. Both strings start the same, but the actual value is missing the following trailing characters: smuggle-it ...