fix(mat-line): fix layout bug (#19809)

* use `flex: auto` instead of `width: 100%` to allow `.mat-list-text`
  element to take up the available space

Author: Sam Severance

src/material/core/style/_list-common.scss

@@ -37,7 +37,7 @@
 
   display: flex;
   flex-direction: column;
-  width: 100%;
+  flex: auto;
   box-sizing: border-box;
   overflow: hidden;