Fixed LocationManagerService position receiver monkey killer

the receiver getter may return null if binder linkToDeath
happens. The returned null will then be added into the
receiver list unchecked, and later used repeatedly when
info needs to be retrieved, all unchecked against null.

This change only adds receiver obj to the list when it
is not null. Also fixes some "receiver" typos.

Change-Id: I6f383964c852b88e90d50e19a895934590e29f76