Improve <amp-analytics type="googleanalytics" default configuration

## Describe the new feature or change to an existing feature you'd like to see

https://github.com/ampproject/amphtml/blob/master/extensions/amp-analytics/0.1/vendors/googleanalytics.json provides basic default configuration. But, it looks like it can be improved to provide 'basic' minimal configuration.

1. For `pageview`, it is having `requests`, but not `triggers`. I think, below default `triggers` or better can be included by default:
```json
"triggers": {
"default pageview": {
"on": "visible",
"request": "pageview",
"vars": {
"title": "${title}" // OR URL?? OR Not needed?
}
}
```
2. Similarly for `social`:
```json
"triggers": {
"tweetClick" : {
"on": "click",
"selector": 'a[href*="twitter.com/share"], a[href*="twitter.com/intent"]',
"request": "social",
"vars": {
"socialNetwork": "twitter",
"socialAction": "tweet",
"socialTarget": "${ampdocUrl}"
}
}

"triggers": {
"fbShareClick" : {
"on": "click",
"selector": 'a[href*="facebook.com/sharer"]',
"request": "social",
"vars": {
"socialNetwork": "facebook",
"socialAction": "share",
"socialTarget": "${ampdocUrl}"
}
}

```

## Reasons

1. Minimal default setup can avoid unnecessary handcoding (sometimes wrong) configurations. For example, may refer to Economic Time's AMP version (Indian Newspaper site)
2. In case of `pageview`, `triggers` is missing. If someone is not too technical, may completely miss it.

## Additional context

Coming from #26691

Start with extensions/amp-analytics/0.1/vendors/googleanalytics.json and review the related context in issue #26691. Compare the proposed pageview and social defaults with the existing vendor configuration and determine the intended minimal behavior. Done means the agreed default configuration is represented consistently in that file and its behavior is validated.

Written by the indexing model from the issue text.