Make the gr-router look for `gr-app` instead of `#app`

We'd like to change the id of the gr-app element to not cause SOY
warnings. Currently, gr-router hardcodes this id. This change removes
that hard-coding therefore making the id of gr-app freely changeable in
the soy template served by the backend.

Change-Id: I15f3bbfd1600f47bd18f2cc30024712c4ff4e6ff

polygerrit-ui/app/elements/core/gr-router/gr-router.ts

diff --git a/polygerrit-ui/app/elements/core/gr-router/gr-router.ts b/polygerrit-ui/app/elements/core/gr-router/gr-router.ts
index 64771f5..2680a31 100644
--- a/polygerrit-ui/app/elements/core/gr-router/gr-router.ts
+++ b/polygerrit-ui/app/elements/core/gr-router/gr-router.ts
@@ -265,7 +265,7 @@
 // custom element having the id "app", but it is made explicit here.
 // If you move this code to other place, please update comment about
 // gr-router and gr-app in the PolyGerritIndexHtml.soy file if needed
-const app = document.querySelector('#app');
+const app = document.querySelector('gr-app');
 if (!app) {
   console.info('No gr-app found (running tests)');
 }