[APM] Enable watcher error reports fails with permissions error in Cloud 7.7

[danielharada]

Describe the bug
The "Enable watcher error reports" action fails with pop-up saying "Watch creation failed. Make sure your user has permission to create watches." in version 7.7 on Cloud instances. This was done with the elastic super user. I confirmed that this user was able to create watches via API that successfully sent an email to the same address used in the APM watcher.

I tested the same action in 7.6.2 and 7.5.2, both succeed following the same steps. I only tested Cloud instances, I have not tested any local versions.

In my Chrome network tab I see that the response to the call to /api/watcher/watch/apm-b1cf90c4-6e06-47ed-973e-1f39175d3694 is {"statusCode":400,"error":"Bad Request","message":"[request body.isNew]: expected value of type [boolean] but got [undefined]"}.

Javascript console shows the following error when executing this action:

PUT /api/watcher/watch/apm-b1cf90c4-6e06-47ed-973e-1f39175d3694 400 (Bad Request)
Error: Bad Request
    at Fetch._callee3$ (commons.bundle.js:3)
    at l (kbn-ui-shared-deps.js:288)
    at Generator._invoke (kbn-ui-shared-deps.js:288)
    at Generator.forEach.e.<computed> [as next] (kbn-ui-shared-deps.js:288)
    at asyncGeneratorStep (commons.bundle.js:3)
    at _next (commons.bundle.js:3)

To Reproduce
Steps to reproduce the behavior:

1. Setup APM in Elasticsearch Service deployment running version 7.7
2. Go to Kibana > APM > Click into your service > Integrations > Enable watcher error reports
3. Leave default Trigger and Condition, enable Send Email action and enter valid email. Click "Create watch"
4. See error

Expected behavior
Watch successfully created. This happens in 7.5.2 and 7.6.2

Actual Behavior
Pop-up saying "Watch creation failed. Make sure your user has permission to create watches."
400 error in Kibana logs, e.g.:
PUT /api/watcher/watch/apm-3bde3644-8490-489c-ab88-5f33ddcdcd0d 400 53ms - 9.0B
No errors in ES logs, nothing shown in ES proxy logs

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)