plugins: Enable message_waiting in ril

Enable the message_waiting atom in the ril plugin.
This exposes the MessageWaiting interface which will
allow notification when new voicemails arrive.

Note -- actual notification of messages received has not yet been tested as it may require changes to SIM IO.  That said, as this interface exposes the VoiceMailboxNumber, which then can be used by the Dialer app to access Voicemail, I've submitted this now.

This has been tested using image r162 on both mako and maguro.  Sanity checks were run to ensure that this has no adverse affects on start-up for both unlocked and PIN-locked phones, and basic voice call tests were performed.