Fix SkShader leak for Gradient VectorDrawable and test

This CL fixes a SkShader leak in VD when applying local matrix
to the shader. Specifically, the usage of newWithLocalMatrix(...)
increments the shader's ref count in every draw() call for
Gradient VectorDrawable, whereas there's no balancing call to
decrement the ref count in draw(). In this CL, we assume
the ownership of the shader returned from newWithLocalMatrix(...)
to ensure the correct ref count management.

Also, add test to verify that shader is no longer being leaked

BUG: 32067647
Test: this CL

Change-Id: Ic15fe46cde06a73d81b44e2d3c56b51907344cc0