Align immediate defaults #539

immediate runs the callback or reads the value once at subscription time. useKeyboardHeight and useAvoidKeyboard default it to true, while subscribeKeyboardHeight, which useKeyboardHeight calls, defaults it to false. useVisibilityEvent and useInterval default it to false.

Use one rule. Subscriptions that return state default to true, because without it the value stays at its initial 0 until the next resize or scroll event. Event callbacks default to false, because an extra call on mount is unexpected. The only change is subscribeKeyboardHeight, whose default becomes true, so its callback runs once right after subscribing. Update docs and tests.

Waits for #531.