fix(expansion): fix expansion .mat-expansion-panel-header styles

* Add margin to `.mat-content` element to account for consumer using
  `mat-expansion-panel` components with the toggle hidden alongside
  `mat-expansion-panel` components where the toggle is visible
* Adjust sizing of `.mat-expansion-panel-header-title` and
  `.mat-expansion-panel-header-description` elements so that the width
  of each respective element is uniform across multiple adjacent
  `mat-expansion-panel` elements
* Fixes #20002

Author: Sam Severance

src/components-examples/material/expansion/expansion-expand-collapse-all/expansion-expand-collapse-all-example.css

@@ -2,11 +2,6 @@
   padding-bottom: 20px;
 }
 
-.example-headers-align .mat-expansion-panel-header-title,
-.example-headers-align .mat-expansion-panel-header-description {
-  flex-basis: 0;
-}
-
 .example-headers-align .mat-expansion-panel-header-description {
   justify-content: space-between;
   align-items: center;

src/components-examples/material/expansion/expansion-steps/expansion-steps-example.css

@@ -1,8 +1,3 @@
-.example-headers-align .mat-expansion-panel-header-title,
-.example-headers-align .mat-expansion-panel-header-description {
-  flex-basis: 0;
-}
-
 .example-headers-align .mat-expansion-panel-header-description {
   justify-content: space-between;
   align-items: center;

src/material/expansion/expansion-panel-header.scss

@@ -45,12 +45,23 @@
   flex: 1;
   flex-direction: row;
   overflow: hidden;
+
+  // width of .mat-expansion-indicator::after element
+  &:only-child {
+    margin-right: 8px;
+
+    [dir='rtl'] & {
+      margin-right: 0;
+      margin-left: 8px;
+    }
+  }
 }
 
 .mat-expansion-panel-header-title,
 .mat-expansion-panel-header-description {
   display: flex;
   flex-grow: 1;
+  flex-basis: 0;
   margin-right: 16px;
 
   [dir='rtl'] & {