whether a physical sensor needed to be active or not was managed by
a simpe reference counter; unfortunatelly nothing prevented it to
get out of sync if a sensor was disabled more than once.
sensorservice already maintainted a list of all the "clients"
connected to a physical sensor; we now use that list to determine if
a sensor should be enabled. This can never be "out-of-sync" since
this is the only data structure linking a sensor to a user of that
sensor.
also removed the isEnabled() method, which was never used and
implemented wrongly (since it didn't take into account that a sensor
could be disabled for a client but not of another).
Change-Id: I789affb877728ca957e99f7ba749def37c4db1c7
50b66767f6