m3: enable GradientDrawable to draw round cap arc

- Add a private Shape type(ARC) to the GradientDrawable.
- Add a new attr strokeCap to GradientDrawable.

The ARC shape also draws ring shape drawable but allows rounded stroke
line cap through a new private attr, strokeCap.

Test: GradientDrawableTest
Flag: com.android.graphics.flags.gradient_drawable_shape_rounded_cap
Bug: 380000245
Change-Id: I358acb9855b4feccf39c40668e48a9626f32fb1c