Upgrade to Ink 1.1.0-alpha03

[maxmmitchell]

Motivated by the programmatic APIs, which are now publicly available as of 1.1.0-alpha03. With this change we can migrate existing code to leverage the programmatic APIs for custom brush construction, rather than using proto builders as we do now.

• Bump AndroidX Ink version to 1.1.0-alpha03, the latest available version as of this PR.
• Update proto file to latest available version from Ink's open source C++ repo. Not strictly necessary for this version bump, but good to do regularly anyway.
• Update code where necessary to remove references to the now-nonexistent ExperimentalInkCustomBrushAPI opt-in annotation. Custom brushes are officially out of the experiemental API phase, and so this annotation no longer exists.

[gemini-code-assist]

Code Review

This pull request updates the ink library dependency from 1.1.0-alpha02 to 1.1.0-alpha03 in gradle/libs.versions.toml. Consequently, the ExperimentalInkCustomBrushApi annotation has been promoted or is no longer required, leading to the removal of its imports and @OptIn annotations across multiple UI and ViewModel files. Additionally, formatting cleanups were applied to DrawingCanvasViewModel.kt, copyright headers were updated in CustomBrushes.kt, and new color function fields (hue_offset_radians, saturation_multiplier, luminosity_offset) along with descriptive comments were added to the brush_family.proto file. I have no feedback to provide on these changes.