Prevent mEventCache UAF in SensorEventConnection

Since there is no check to see if SensorEventConnection has been
destroyed, the mEventCache pointer can still be used even after it
was freed.

Bug: 168211968
Test: Run test code that attempts to enable a sensor after destroying
the SensorEventConnection, and verify no system_server crash occurs.

Change-Id: Ia9275b7cc574df371cdb2e1b80c6699df193b580
Merged-In: Ia9275b7cc574df371cdb2e1b80c6699df193b580
(cherry picked from commit 09052cc6)