Scribble method namespace

[justinmc]

In the framework (https://github.com/flutter/flutter/pull/104128), the scribble feature is being moved out of TextInputClient and into its own class. This PR updates the naming of the platform channel methods to match their new names in the framework.

[flutter-dashboard[bot]]

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[chinmaygarde]

Depends on https://github.com/flutter/flutter/pull/104128

[justinmc]

@cyanglaz Good idea about doing this in stages. I've added back the TextInputClient usages so this PR now has both. I'll come back and remove them after this and the framework PR land.