refactor: Simplify Engine constructor

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.

This relates to MetaMask/mobile-planning#1015