fix(android): fix headless crash with null ReactContext

very tricky to make sure react-native is initialized during
headlessJS startup so you don't beat it in the race to asking
for a ReactContext before it exists

- startup is event-driven so you have to attach listeners
- startup has to run on main thread so you have to post runnables
- multiple tasks may come in while starting up, you must queue / drain
- startup objects / methods differ across react-native new / old arch

get all that right, and you'll have a valid ReactContext, without
dropping any tasks that needed it before react-native was initialized