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
f571003794