fix(button-toggle): allow toggle group elevation to be overwritten

Allows for consumer to specify a custom elevation on a `mat-button-toggle-group`.

Relates to #7264.

Author: crisbeto

src/lib/button-toggle/_button-toggle-theme.scss

@@ -8,7 +8,7 @@
   $background: map-get($theme, background);
 
   .mat-button-toggle-standalone, .mat-button-toggle-group {
-    @include _mat-theme-elevation(2, $theme);
+    @include _mat-theme-overridable-elevation(2, $theme);
   }
 
   .mat-button-toggle {

src/lib/button-toggle/button-toggle.scss

@@ -11,9 +11,7 @@ $mat-button-toggle-border-radius: 2px !default;
   position: relative;
   display: inline-flex;
   flex-direction: row;
-
   border-radius: $mat-button-toggle-border-radius;
-
   cursor: pointer;
   white-space: nowrap;
   overflow: hidden;