Support stylus handwriting input on Android

[justinmc]

Recently, support for Apple Pencil support was added to Flutter via the "Scribble" feature. We should also support stylus handwriting input on Android as well, using a common set of framework APIs.

How?

Android's InputMethodManager class already has support for stylus handwriting. We'll need to tie these into the framework similarly to how it's done for iOS.

Some relevant methods:

• InputMethodManager.startStylusHandwriting
• View.setAutoHandwritingEnabled

Will Flutter's Scribble API need to change?

Ideally not, but it's unclear to me whether or not we'll have to. We want to make everything available in the framework that we can and not produce a "leasts common denominator" API.

References

Scribble framework PR: #75472
Scribble engine PR: flutter/engine#24224

Stylus input on other platforms (not necessarily handwriting):
Linux: #63209
Windows: #102836