Remove onRemoved callbacks

Removes the necessity for LMS to call back into client code to inform
the client that a location listener has been removed. This is replaced
with weak references instead, which allows the garbage collector to
handle cleanup instead. This simplifies LMS implementation and makes it
less buggy.

Also includes some rewrites of getCurrentLocation to use a dedicated
AIDL interface now that the onRemoved channel is no longer available,
as well as various other minor refactors.

Test: manual + presubmits
Change-Id: Ic61ce278c44e23bc5da5b703f89f6e656e89dea2