Fix NPE when removing PIN locked SIM during E911 call

There is the possibility of mCallback being null while
the Keyguard still has a PhoneStateListener registered
for onSimStateChanged callback. This can cause a NPE.

Bug: 66986066
Test: Manual, PIN/PUK locked SIM
Change-Id: I799c9521f787e800e259c3bdce9edee3f844f5f3