assistant: update input text hint color

Increase the contrast for a11y.

Bug: 910560
Test: locally build and run
Change-Id: Iedd3fc2ffa45ca6c637afcc046ff0c35a826f9ab
Reviewed-on: https://chromium-review.googlesource.com/c/1368110
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614807}

ash/assistant/ui/assistant_ui_constants.h

@@ -25,9 +25,8 @@ constexpr int kMarginDip = 8;
 constexpr int kUiElementHorizontalMarginDip = 32;
 
 // Typography.
-constexpr SkColor kTextColorHint = gfx::kGoogleGrey500;
+constexpr SkColor kTextColorHint = gfx::kGoogleGrey700;
 constexpr SkColor kTextColorPrimary = gfx::kGoogleGrey900;
-constexpr SkColor kTextColorSecondary = gfx::kGoogleGrey700;
 
 // TODO(dmblack): Move the other constants into ash::assistant::ui.
 namespace assistant {