Add APIs to make Android Path queryable and more useful

The underlying Skia implementation of Path has APIs for
querying the data in a path (via SkPath::Iter). It also exposes
the ability to add conic sections (conicTo()), which is useful
since a Path can contain conics (even if it was not created with
conics directly).  And it adds a method to get the generationId
from a Path, used to see whether a Path changed since the
last time that Id was retrieved. Finally, it exposes a simple
interpolate() function to allow linear interpolation between
two paths.

This CL adds PathIterator to enable querying Path data, and
adds conicTo, isInterpolatable(), and interpolate() to Path,
all of which wrap the underlying, existing Skia functionality.

Bug: 157391114
Bug: 223586753
Test: Added PathIteratorTest plus new tests on PathTest
Change-Id: Iddadfde24dd75ac04d0a2f3ebca767613af25360