Skip RAT type listener registration if IMSI is not available

Currently, if SIM is inserted but IMSI is not available, such
as SIM PIN locked state. Information of such SIM will still
be available but IMSI is not. Which makes
NetworkStatsSubscriptionMonitor failed to store IMSI locally
for later RAT type query. Hence, NETWORK_TYPE_UNKNOWN is
always returned for such SIM.

Skip the registration until the IMSI is available. This is safe
since there will be another onSubscriptionsChanged event when
that happens.

Test: enable SIM PIN and manually test
Test: atest NetworkStatsSubscriptionsMonitorTest#testSubscriberIdUnavailable
Test: ./out/host/linux-x86/bin/statsd_testdrive 10082
Bug: 160941101
Merged-In: I408379b3c432d9e62e0837d6b4f6551cc7838e29
Change-Id: I408379b3c432d9e62e0837d6b4f6551cc7838e29
(cherry-picked from ag/12400327)