Log Telephony Event on receiving RIL_UNSOL_MODEM_RESTART

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.

Test: Modified vendor RIL to send modem restart unsol message and
verified that the event is logged, added a test in TelephonyMetricsTest
and ensured all TelephonyMetricsTest tests pass

Test: Manual
Merged-In: I6bc045341be59ac45cc2a8dd83bdbb3748352c0d
Change-Id: I6bc045341be59ac45cc2a8dd83bdbb3748352c0d