Add null check to InputMonitor.notifyANR

Followup to 7402ddf9b45efc9616676205534ce7b4fc3a13e6

Test: Start IME, induce ANR in Settings, verify /data/anr/traces.txt
Change-Id: I4ad1d48cc73ca1a4332414ff50a160c67979feef
Fixes: 36730136
2 files changed