Fix the gradle templates so that they default to values rather than throw build exceptions. (#19916)

When creating a new project, the build fails with an error similar to:

"versionCode not found. Define flutter.versionCode in the local.properties file."

This puts developers in the untenable situation of having to edit a file with local paths, but being unable to check it in.

This changes the templates to default to using versionCode 1 and version "1.0" if they are not defined in the local.properties file.

Fixes #18983.