Support dynamically update IMSI

Currently, NetworkStatsSubscriptionsMonitor will skip the
listener registration until the IMSI is available to deal
with SIM PIN locked case. However, this solution can only
handle the case that IMSI changes from/to null. And it also
relies on the assumption that IMSI never changes for a subId.

Thus, support dynamically update IMSI to handle IMSI changes
more robustly.

This patch also address leftover comments at ag/12400327.

Test: enable SIM PIN and manually test
Test: atest NetworkStatsSubscriptionsMonitorTest#testSubscriberIdUnavailable
Test: atest NetworkStatsSubscriptionsMonitorTest#testSubscriberIdChanged
Test: ./out/host/linux-x86/bin/statsd_testdrive 10082
Bug: 160941101

Change-Id: I625a5b10ee4806f6fee99c2d9d6c5e7977ff785e