PolyGerritIndexHtml: Rename gr-app id to fix soy warning

After closure-templates:soy update to recent release we are seeing this
warning:

  WARNING: warnings during Soy compilation
  com/google/gerrit/httpd/raw/PolyGerritIndexHtml.soy:160: warning:
  Html id attributes should not be valid JavaScript identifiers,
  consider hyphenating the id.
  160:   <gr-app id="app"></gr-app>{\n}
                    ~~~~~
  1 warning

Bug: Issue 15621
Change-Id: Ice695c47c86e2d8844089de0c4cc110a576995b9
diff --git a/resources/com/google/gerrit/httpd/raw/PolyGerritIndexHtml.soy b/resources/com/google/gerrit/httpd/raw/PolyGerritIndexHtml.soy
index 11717fb..9428c2f 100644
--- a/resources/com/google/gerrit/httpd/raw/PolyGerritIndexHtml.soy
+++ b/resources/com/google/gerrit/httpd/raw/PolyGerritIndexHtml.soy
@@ -168,7 +168,7 @@
   <link rel="stylesheet" href="{$staticResourcePath}/styles/main.css">{\n}
 
   <body unresolved>{\n}
-  <gr-app id="app"></gr-app>{\n}
+  <gr-app id="pg-app"></gr-app>{\n}
 
   // Load gr-app.js after <gr-app ...> tag because gr-router expects that
   // <gr-app ...> already exists in the document when script is executed.