fix(material/tabs): strong focus indicator not visible when tab header has a background (angular#21402)

`mat-tab-group` has the ability to theme both the foreground and background. When
the background color is different from the default, the strong focus indicator blends
into it, making it invisible.

These changes invert the color of the indicator so it stands out.

Author: crisbeto

src/material-experimental/mdc-tabs/_tabs-theme.scss

@@ -80,7 +80,9 @@
       @include mdc-theme-prop(background-color, $foreground-color);
     }
 
-    .mdc-tab-indicator__content--underline, .mat-mdc-tab-header-pagination-chevron {
+    .mdc-tab-indicator__content--underline, .mat-mdc-tab-header-pagination-chevron,
+    .mat-mdc-focus-indicator::before,
+    .mat-mdc-focus-indicator::before {
       @include mdc-theme-prop(border-color, $foreground-color);
     }
   }

src/material/tabs/_tabs-theme.scss

@@ -116,7 +116,9 @@
   }
 
   // Set pagination chevrons to contrast background
-  > .mat-tab-header-pagination .mat-tab-header-pagination-chevron {
+  > .mat-tab-header-pagination .mat-tab-header-pagination-chevron,
+  > .mat-tab-links .mat-focus-indicator::before,
+  > .mat-tab-header .mat-focus-indicator::before {
     border-color: mat-color($background-color, default-contrast);
   }