Report device storage status (for SMS) to RIL.

Indicate SMS memory available/not available based on DEVICE_STORAGE_OK/FULL
intents, and immediately NAK with MEMORY_FULL if we've received a
DEVICE_STORAGE_FULL notification.

This may end up being a short term solution, as the messaging app is
getting re-worked in Eclair, which may change the way we want to handle
memory full reporting (ie, more app involvement).