refactor: Simplify Engine constructor

[Gudahtt]

Development & PR Process

1. Follow MetaMask Mobile Coding Standards
2. Add release-xx label to identify the PR slated for a upcoming release (will be used in release discussion)
3. Add needs-dev-review label when work is completed
4. Add the appropiate QA label when dev review is completed
   • needs-qa: PR requires manual QA.
   • No QA/E2E only: PR does not require any manual QA effort. Prior to merging, ensure that you have successful end-to-end test runs in Bitrise.
   • Spot check on release build: PR does not require feature QA but needs non-automated verification. In the description section, provide test scenarios. Add screenshots, and or recordings of what was tested.
5. Add QA Passed label when QA has signed off (Only required if the PR was labeled with needs-qa)
6. Add your team's label, i.e. label starting with team- (or external-contributor label if your not a MetaMask employee)

Description

The Engine class is riddled with type errors, which has been making the controller updates difficult. In attempting to address these errors, one of the first obstacles was the conditional constructor. The types for the class would be a lot simpler if we could assume that certain properties were always initialized, but the condition prevents that.

The condition was meant to ensure that only one global Engine instance was ever constructed. This check has been moved to the init function at the end of the file instead. This has the same effect, as that function is the only place the Engine class is constructed (it's not exported). This is much easier for TypeScript to understand.

Issue

This relates to https://github.com/MetaMask/mobile-planning/issues/1015

Checklist

• There is a related GitHub issue
• Tests are included if applicable
• Any added code is fully documented

[Gudahtt]

For reviewers, I'd strongly recommend using the "Hide whitespace" option in the diff view. Most of the changes are due to the constructor indentation change.

[codecov-commenter]

Codecov Report

Patch coverage: 66.30% and project coverage change: -0.01% ⚠️

Comparison is base (61d1eb3) 32.83% compared to head (b6739b5) 32.82%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7091      +/-   ##
==========================================
- Coverage   32.83%   32.82%   -0.01%     
==========================================
  Files        1001     1001              
  Lines       26709    26706       -3     
  Branches     2096     2095       -1     
==========================================
- Hits         8769     8766       -3     
  Misses      17524    17524              
  Partials      416      416              

Files Changed	Coverage Δ
app/core/Engine.ts	57.21% <66.30%> (-0.63%)	⬇️

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[Gudahtt]

E2E smoke test run: https://app.bitrise.io/app/be69d4368ee7e86d/pipelines/06720f8c-3ee7-456f-9131-3048a7358b6b

[github-actions]

CLA Signature Action: All authors have signed the CLA. You may need to manually re-run the blocking PR check if it doesn't pass in a few minutes.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

91.7% Coverage
0.0% Duplication

The version of Java (11.0.20) you have used to run this analysis is deprecated and we will stop accepting it soon. Please update to at least Java 17.
Read more here

[Cal-L]

LGTM