Incorporate new API council comments

- Split getStatus() into onGetSummary() and onGetEnabled()

- Call them on app's UI thread

- Allow runtime exceptions to propagate up

- Make a couple of more methods final to prevent subclasses from playing
around with the intent

- Remove explicit timing requirement from javadoc

- Mention that this will be restricted to system-image apps (will be
enforced by the actual settings code)

- b/10461474

Change-Id: Id22dd7a707c05de396ae4c5810e839ca734714c0