Some cleanup on the use of Soy index:
* The canonical-path property of GR-APP is removed because it did not
initialize at the right time and a top-level global is used instead.
* Only set a top-level global for canonical path if it is not the empty
string. In this way, the index document does not include an inline
script when it is not needed.
* Give the index Soy template a more descriptive name. index.html.soy
was too generic, and the intermediate period would cause problems with
some internal build tooling.
Bug: Issue 5919
Change-Id: I0bf5964b17cb4f4f9df150b0c6abe8ae1e49f22b
9fd72e0615