Unify all use snippet toggle labels

## Bug Description

We're using different labels for the same setting for each module:

**Analytics**
- Code snippet
- Place Analytics code

**AdSense**
- AdSense Code: The AdSense code has been placed on your site
- Let Site Kit place AdSense code on your site

**Tag Manager**
- Tag Manager Code Snippet: Snippet is inserted
- Let Site Kit place code on your site

Consider making labels uniform, with the below:
- **Before editing** (on the connected services page): Code snippet: Snippet is/is not inserted
- **When editing**: Let Site Kit place code on your site

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## Acceptance criteria

*

## Implementation Brief

* [ ]

### Test Coverage

*

## QA Brief

*

## Changelog entry

*

Start by locating the Analytics, AdSense, and Tag Manager connected-services and editing controls that display the labels described in this issue. Compare their current wording with the proposed uniform labels; done when each module uses the specified connected-services and editing labels.

Written by the indexing model from the issue text.