refactor gradient stage names

This is just a name refactor and I'm happy to delay it until we're done
with the current wave of gradient CLs.  The main ideas:

  - we use the "linear_gradient" stages for all gradients,
    so cut the "linear" and just call them "gradient";

  - remind ourselves that the 2-stop stage requires even spacing, i.e.
    stops at 0 and 1.  This name should harmonize with Herb's new
    general evenly spaced gradient stage, currently
    "evenly_spaced_linear_gradient", and after it lands and I rebase,
    "evenly_spaced_gradient"

  - remind ourselves which polar coordinate xy_to_polar_unit returns,
    the angle.

Change-Id: I0fd0c8bd4c1ead7d2d0fff45a199d318b71f34ac
Reviewed-on: https://skia-review.googlesource.com/16500
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Reed <reed@google.com>

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