Fix potential NPE in SubscriptionMonitor

Move the callback registration to after all the member variables have
been set up, so that a potential call to the callback doesn't end up
with uninitialized member variables.

Also clean up some imports

Fixes: 134102514
Test: manual
Change-Id: I202b00c58e11c77b6b31f0b5e17e6b9eee25ef33