Fix KotlinNullableVariableCastToNonnullType in DefaultReactHost.kt (#55406)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/55406

Refactored getDefaultReactHost to prove that reactHost is never null at return time, eliminating the need for checkNotNull.

The original code used `checkNotNull(reactHost)` which was a runtime assertion. The refactored code uses a pattern that proves null-safety at compile time:
1. Early return with `?.let` if reactHost is already initialized
2. Create the new ReactHostImpl in a local `val newReactHost` (which compiler knows is non-null)
3. Assign to the class field and return the local variable directly

This approach eliminates the runtime check while maintaining the same singleton behavior.

changelog: [internal] internal

Reviewed By: javache

Differential Revision: D91820164

fbshipit-source-id: 4e550569e8473b549ba0debb701991a260a8dd04