Add ContentProvider for apps to read received SMS cell broadcasts.

The CellBroadcastReceiver app will allow apps with the new permission
"android.permission.READ_CELL_BROADCASTS" to read previously received
cell broadcast messages from a new ContentProvider database at URI
"content://cellbroadcasts". This will enable third parties to provide
additional information to users in the event of emergencies without
delaying or interfering with the initial system alert dialog to warn
the user when the alert is received.

Includes a new android.telephony.CellBroadcastMessage class which
can be instantiated from the Cursor retrieved from the ContentProvider.
This was previously a part of the CellBroadcastReceiver app, but can now
be used by third party apps with read permission on the ContentProvider.

Change-Id: I2c31f62b63c050c7946de2d81c28a5f4dc6f00b0