Add the missing unregisterCallback call

The sensor reading thread continues producing
event and refers to the already destroyed
callback object which causes a crash in
SensorsHidlEnvironmentBase::addEvent.

Bug: 153754380
Test: atest VtsHalSensorsV2_0TargetTest
Signed-off-by: Roman Kiryanov <rkir@google.com>
Change-Id: Id7fb72c42dc67f89ceef93d937241e1584b853e7