cros: Remove the start search page in KSV.

This cl updates the following views and behaviors in Keyboard Shortcut
Viewer (KSV).
1. Remove the start search page.
2. Add Esc and Back key supports and update behavior with Backspace key.
3. Unbold text in no search results page.
4. Make the search bar 2px longer at both sides to align with text.
5. Update strings and tooltips of buttons.
6. Make the window 32px wider in order to put all shortcuts in one line.

Bug: 768932
Test: Tested on device.
Change-Id: Ief361b80b8326dfd8b3aea1b0cd17a42040be95d
Reviewed-on: https://chromium-review.googlesource.com/942687
Commit-Queue: Tao Wu <wutao@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540372}

ash/components/shortcut_viewer/vector_icons/BUILD.gn

@@ -16,8 +16,6 @@ aggregate_vector_icons("ksv_vector_icons") {
     "ksv_search_close.icon",
     "ksv_search_no_result.1x.icon",
     "ksv_search_no_result.icon",
-    "ksv_search_start.1x.icon",
-    "ksv_search_start.icon",
     "ksv_separator_plus.1x.icon",
     "ksv_separator_plus.icon",
     "ksv_arrow_down.1x.icon",

ash/components/shortcut_viewer/views/keyboard_shortcut_item_view.cc

@@ -148,8 +148,9 @@ void KeyboardShortcutItemView::MaybeCalculateAndDoLayout(int width) const {
   // The width of |description_label_view_| and |shortcut_label_view_| as a
   // ratio of its parent view's width. The unused width is to have some spacing
   // in between the two views.
-  constexpr float kDescriptionViewPreferredWidthRatio = 0.32f;
-  constexpr float kShortcutViewPreferredWidthRatio = 0.65f;
+  // These values are chosen to put all the bubble views in one line.
+  constexpr float kDescriptionViewPreferredWidthRatio = 0.29f;
+  constexpr float kShortcutViewPreferredWidthRatio = 0.69f;
   const int description_view_preferred_width =
       width * kDescriptionViewPreferredWidthRatio;
   const int shortcut_view_preferred_width =