[webview_flutter] Fix wrong main thread checking condition.

[amirh]

Description

The Android JavaScriptChannel wasn't checking correctly if messages are
received on the main thread and took the branch that assumes we are on
the main thread.

flutter/engine#8830 causes this behavior to fail
(which is good). So now we can also add an e2e test that fails with the
buggy behavior.
Replace this paragraph with a description of what this PR is doing. If you're modifying existing behavior, describe the existing behavior, how this PR is changing it, and what motivated the change.

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[iskakaushik]

LGTM