Add RIL_UNSOL_MODEM_RESTART message

RIL_UNSOL_MODEM_RESTART will be sent by vendor RILs when it detects
a modem restart along with the reason for restart which could be a
crash signature if the modem restarted due to a crash or a some
other string such as a user-initiated restart or AT command
initiated restart.

Change-Id: Idf08e20a49e82d40f6aa0854d23384c35c2efc3a