Improve Minerva drop-shadows in dark-mode

Converts several drop-shadows to use standard Codex values:
- `.search-box .search` uses `@box-shadow-drop-small`
- `.drawer, .toast` uses `@box-shadow-drop-xx-large`
- `.toggle-list__list--drop-down` uses `@border-color-base`

Bug: T364425
Change-Id: Id00e525655d3a7138aa65f424fd6ab2ff82290c8

includes/Skins/ToggleList/DropdownList.less

@@ -8,7 +8,8 @@
 	overflow-y: auto;
 	// The menu floats over content but below overlays.
 	z-index: @z-index-drawer;
-	box-shadow: 0 5px 17px 0 rgba( 0, 0, 0, 0.24 ), 0 0 1px #a2a9b1;
+	// Two box-shadows: first creates a drop-shadow, second acts as a subtle border.
+	box-shadow: 0 5px 17px 0 rgba( 0, 0, 0, 0.24 ), 0 0 1px @border-color-base;
 	visibility: hidden;
 	opacity: 0;
 	transform: translateY( -8px );

resources/skins.minerva.base.styles/ui.less

@@ -112,7 +112,7 @@ main {
 	border: @border-width-base @border-style-base var( --border-color-inverted );
 	border-radius: @border-radius-base;
 	padding: @padding-vertical-search 0 @padding-vertical-search @padding-start-search-icon;
-	box-shadow: 0 1px 1px rgba( 0, 0, 0, 0.05 );
+	box-shadow: @box-shadow-drop-small;
 	// Keyboard focus is taken care of below at `.search:focus`.
 	outline: 0;
 	transition-property: border-color, box-shadow;

skinStyles/mobile.startup/drawers.less

@@ -36,7 +36,7 @@
 .toast,
 .drawer {
 	box-sizing: border-box;
-	box-shadow: 0 -1px 8px 0 @box-shadow-color-base;
+	box-shadow: @box-shadow-drop-xx-large;
 	word-wrap: break-word;
 	// don't use visibility: hidden in old browsers that don't support animations
 	display: none;