Update z-index of main header

Because of the sticky section headers in the change list view, the main
header z-index has to be increased to be greater than 1 so that the
dropdowns appear on top of the sticky headers.

Bug: Issue 8512
Change-Id: I7fad70047ae08bf599f38d67bba6266845134c28
diff --git a/polygerrit-ui/app/elements/gr-app.html b/polygerrit-ui/app/elements/gr-app.html
index 7834949..2f2aa51 100644
--- a/polygerrit-ui/app/elements/gr-app.html
+++ b/polygerrit-ui/app/elements/gr-app.html
@@ -86,8 +86,9 @@
         background-color: var(--header-background-color);
         padding: 0 var(--default-horizontal-margin);
         position: sticky;
-        /* Needed for the menu dropdowns to display */
-        z-index: 1;
+        /* Needed for the menu dropdowns to display on top of the sticky
+          headers with z-index of 1.*/
+        z-index: 2;
       }
       footer {
         /* The combination of left, width, position allow this to be