Quickfix: Run Realm.App._clearAppCache() on load.

[steffenagger]

This PR executes Realm.App._clearAppCache() on load, to re-matchup C++ & JS environments for RN hot-reloading.

This closes #3668

[kraenhansen]

I don't know if this is a huge problem, but it seems we're already clearing this cache natively on iOS from react-native/ios/RealmReact/RealmReact.mm#L267 (which calls src/jsc/jsc_init.cpp#L57-L62).

To avoid calling it twice, we might either want to remove it from the native code entirely or add it to the Android native code instead of calling it from JS.

[kneth]

It might be safer to do it when you load Realm, but I agree with @kraenhansen that we shouldn't do it twice.

[kraenhansen]

Okay. I realize now that we're clearing other caches when running on Android here: https://github.com/realm/realm-js/blob/master/src/android/jsc_override.cpp#L137 - we should probably just add the realm::app::App::clear_cached_apps(); after that line.