Diagnostics: Layout and card updates

Screenshot: http://shortn/_AyCiiMx92w

 - Center cards
 - Adjust margin within cards
 - Adjust card width
 - Remove minimum height requirement from cards

Bug: 1125150
Test: browser_tests --gtest_filter=DiagnosticsApp*
Change-Id: I4c4f31182c7c835f6015b15af0f5a3aab64f489d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2530744
Commit-Queue: Michael Checo <michaelcheco@google.com>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827994}

chromeos/components/diagnostics_ui/resources/diagnostics_app.html

 <style include="cr-shared-style diagnostics-shared diagnostics-fonts">
   .diagonstics-cards-container {
+    align-items: center;
     display: flex;
     flex-direction: column;
   }

chromeos/components/diagnostics_ui/resources/diagnostics_card.html

@@ -23,6 +23,7 @@
     display: flex;
     flex-direction: column;
     margin-top: 12px;
+    width: 650px;
   }
 
   .card-header {
@@ -36,7 +37,6 @@
     border: 1px solid var(--diagnostics-border-color);
     border-radius: 14px;
     margin: 10px 0;
-    min-height: 150px;
   }
 
   .routine-container {
@@ -44,7 +44,7 @@
   }
 
   .spacing {
-    margin: 18px 0px;
+    margin: 12px 0px;
     padding-inline-end: 15px;
     padding-inline-start: 15px;
   }