Tests that each Emulation handler keeps track of its sensor overrides

Creating virtual sensors
PASS: Attempting to override sensor already overridden in other session fails

Disconnecting |session3|
PASS: 'accelerometer' is being overridden
PASS: 'gravity' is being overridden
PASS: 'gyroscope' is being overridden

Removing GravitySensor virtual sensor
PASS: 'gravity' is not being overridden
PASS: 'accelerometer' is being overridden
PASS: 'gyroscope' is being overridden

Disconnecting |session2|
PASS: 'accelerometer' is not being overridden
PASS: 'gravity' is not being overridden
PASS: 'gyroscope' is being overridden

Creating a new virtual accelerometer
PASS: setSensorOverrideEnabled() does not fail this time

Disconnection session
PASS: 'accelerometer' is not being overridden
PASS: 'gravity' is not being overridden
PASS: 'gyroscope' is not being overridden