Only send storage intents after boot complete

It is not possible to send a broadcast before the system
boot is completed. If you do it anyway you will get an
IllegalStateException: Cannot broadcast before boot completed.

If a memory card is inserted or removed while the phone is
booting up, there is a risk that the MountService will try to
broadcast a storage intent too early, and cause the device
to crash.

Use FLAG_RECEIVER_REGISTERED_ONLY_BEFORE_BOOT to avoid
problems with too early broadcasts.

Change-Id: Ied36a13d235df37c9788e45a35de40d919ae0cd0