Fix Canvas#drawVertices color blending when no shader is provided

Since https://skia-review.googlesource.com/c/skia/+/473676, Skia will
blend paint and vertex colors when no shader is provided. This fix
mimics the old behavior of ignoring the paint and using the vertex
colors directly when no shader is provided.

Additionally, centralizes some legacy compatability logic of disabling
the shader if no texs array is provided. I think this simplifies the
logic/cognitive overhead of understanding what compatability tweaks are
being made to the call.

Tests added in Iddb4d2a3872b905684ec57c0e039d0bd0ca64a71
Test: atest ExactCanvasTests

Fix: 239398877

Change-Id: Ie9a6389b422cc0c315485d8a8097314cb6706538