feat: make reactHost accessible

[WoLewicki]

Summary:

On bridgeless mode, reactHost is kept in memory even after destroying the DefaultReactNativeHost in brownfield scenario. Since it keeps references to the modules, they are not deallocated, and their initialize methods are not fired again when creating new instance of react-native later. It breaks the behavior of e.g. react-native-screens, which wants to listen for mutations and should get new FabricUIManager: https://github.com/software-mansion/react-native-screens/blob/20b7e83782cd5f79ddd0d61dadc13eeb4db4b258/android/src/main/java/com/swmansion/rnscreens/ScreensModule.kt#L45.

By making the variable public, we can make it null when destroying the RN activity, removing references to everything it kept.

Changelog:

[ANDROID] [FIXED] - make it possible to set reactHost to null.

Test Plan:

It shouldn't change anything since it is just change of visibility in the variable.