Add keyboard background gradients and a purple pink theme

[tgll]

Adds optional support for two-color vertical gradients on the keyboard background, along with a Purple Pink Gradient theme demonstrating the feature.

Changes

• Adds optional theme attributes for the gradient start and end colors
• Draws the gradient behind the keyboard keys
• Adds the Purple Pink Gradient theme
• (Preserves the existing solid-color rendering for themes without gradient attributes)
• (Does not modify keyboard input behavior)

(Tested on a Pixel 8 Pro API 37 emulator)

[Julow]

That's a really good idea!

[Julow]

This is not the right class to define and update _keyboardBackgroundPaint. Paints are computed in Theme.Computed (line 106 in Theme.java).