Last position improvements for GeofenceManager

Use LocationManager.getLastPosition() in GeofenceManager instead of
keeping track of it manually. Keeping track of it in GeofenceManager
doesn't handle the case where we install a fence, and cross it just
after that based on the last position before we installed the fence.

Also shuffle around some code in LocationManagerService to remember the
last position even if there are no UpdateRecords. This is useful in the
GeofenceManager for example.

Bug: 7047435
Change-Id: Ia8acc32e357ecc2e1bd689432a5beb1ea7dcd1c7