[webview_flutter] Disable legacy Android video poster

[lauweijie]

It looks like the chromium team is considering disabling the default video poster feature: https://issues.chromium.org/issues/40755557. This changes it to an empty bitmap for now.

Fixes https://github.com/flutter/flutter/issues/178263.

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[bparrishMines]

Opted out to land in downstream fork for now.

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[reidbaker]

@lauweijie is this ready for review or did you want it to be a draft?

[bparrishMines]

@reidbaker I set this as a draft while we wait to see what the process will be to roll this plugin internally. It's only open still to ensure that we revisit it after a decision. But I'm also open to closing it for now if it is easier for triage teams.

[reidbaker]

I just wanted to make sure that it wasn't accidentally marked as draft.

[stuartmorgan-g]

From triage: @lauweijie are you still planning on updating this to make it an option, so that we can land it upstream?

[lauweijie]

Not currently, I'll come back to this when I have more time to make it configurable as an option. I guess there are two ways to do this: one is to have a boolean option to decide if we should disable the default poster behavior (and override it with the empty bitmap), another option is to allow flutter to provide the bitmap. Happy for someone else to take this up.