Fix bug in getCurrentLocation cancellation

Cancellation was not being propagated to the system server due to a
misunderstanding on how CancellationSignal functions. There is no
noticable impact for the user, as cancellation did properly prevent any
further locations, however this would have resulted in limited extra
power draw since the system server would not have canceled the
associated provider location request. Impact is limited since
getCurrentLocation() was limited to a 30s duration anyways.

Bug: 168666216
Test: manual + presubmits
Change-Id: I5c77db4cc56cce1b984587f65d2bcfb488436bb8