Add configuration to delay the first start of Push until Push.setEnabled(true) is called from JS

[guperrot]

Things to consider before you submit the PR:

• Has CHANGELOG.md been updated?
• Are tests passing locally?
• Are the files formatted correctly?
• Did you test your change with either the sample apps that are included in the repository or with a blank app that uses your change?

Description

To delay start push with this change:

• Add EnablePushInJavascript YES to AppCenter-Config.plist
• Add "enable_push_in_javascript": true to appcenter-config.json
• Call Push.setEnabled(true) from Javascript when developer wants to start Push.

Since this is only for the first time we want to delay Push, the code also remembers the first call to Push.setEnabled(true) and will start Push automatically the next time.

Documentation PR: https://github.com/MicrosoftDocs/appcenter-docs-pr/pull/1000

Requested in #287

[dhei]

I am thinking maybe use this API in TestApp?

[jdnichollsc]

Hi guys,
EnablePushInJavascript is not working for iOS, the event onPushNotificationReceived is not executed in that platform

[guperrot]

@jdnichollsc it works for us in TestApp from this repo but it looks like your issue is caused by something else since you commented on #505.

[jdnichollsc]

@guperrot thanks, I'm reviewing the code of AppCenter but I can't see any issue

[guperrot]

We will investigate and track in #505 even if not related (can still rename the title there when we find the cause).

Also you mentioned in #505 that version 1.8.0 of the SDK also has the issue so that means it's not caused by this new configuration feature and is a older issue.