Add conic support for Path#approximate

Fixes: 37722722
Test: bit CtsGraphicsTestCases:PathTest
Test: bit CtsUiRenderingTestCases:android.uirendering.cts.testclasses.PathTests

Also adds static asserts to path-walking code, to avoid this problem
in the future.

Also adds annotations, since this is public API now.

Change-Id: Ic39b167968b98fd8197be2d0f9aca79949717237