[User Model] Safeguard against activities/services/receivers being called before initialization

[brismithers]

Description

One Line Summary

Safeguard against activities/services/receivers being called before initialization.

Motivation

• Update IOneSignal.initWithContext interface to return whether initialization was successful or not.
• Implementation fails initialization if no app ID is provided and no app ID has been saved.
• Update all activities/services/receivers to check and not proceed when initialization unsuccessful.

Scope

Fixes #1745
Covers all SDK services/receivers/activities initialization flows, which call OneSignal.initWithContext without an appId (as they don't know the appId) on the assumption the appId has already been provided by the external OneSignal.initWithContext. Now these services/receivers/activities can bail out if this is not the case, rather than blow up.

Testing

Manual testing

Test various scenarios related to initializing (or not initializing) the OneSignal SDK and driving the services/receivers/activities that exist within the SDK.

Affected code checklist

• Notifications
  • Display
  • Open
  • Push Processing
  • Confirm Deliveries
• Outcomes
• Sessions
• In-App Messaging
• REST API requests
• Public API changes

Checklist

Overview

• I have filled out all REQUIRED sections above
• PR does one thing
  • If it is hard to explain how any codes changes are related to each other then it most likely needs to be more than one PR
• Any Public API changes are explained in the PR details and conform to existing APIs

Testing

• I have included test coverage for these changes, or explained why they are not needed
• All automated tests pass, or I explained why that is not possible
• I have personally tested this on my device, or explained why that is not possible

Final pass

• Code is as readable as possible.
  • Simplify with less code, followed by splitting up code into well named functions and variables, followed by adding comments to the code.
• I have reviewed this PR myself, ensuring it meets each checklist item
  • WIP (Work In Progress) is ok, but explain what is still in progress and what you would like feedback on. Start the PR title with "WIP" to indicate this.

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