[Decor] Remove unused properties and Tunable

Before adding support for runtime debugging, we can remove the `Tunable`
implementation from ScreenDecorations, as well as the unused fields
declared.

Tunable was added to the screen decor when the assumption was that a
single path was suitable for all rounded corners. Therefore, the only
supported tuning was `SIZE`, which simply changes the size of the
rounded corners. With the proposed changes, this tuning size is no
longer relevant. The CLI that will be implemented can handle arbitrary
paths, viewport sizes, and colors, and does not require registering
entries with the TunerService.

Test: ScreenDecorationsTest
Test: RoundedCornerResDelegateTest
Bug: 285941724
Change-Id: If9f3e28c641590c04d19315970359bed04521bc6