Fix for crash in LocationManagerService

Resolved issue where MockProvider was being cast to
LocationProviderProxy and would cause a fatal exception.

Change-Id: I7c8c83961dd03d59292e773b01995ed6b3ac04c7
CRs-fixed: 619882
Clear cached locations when location providers disabled Bug: 12118307

(cherry picked from commit b084fef18ee1b5d24d15cff37dc3a053933d361e)

Change-Id: Ief869b9e38858092bfa3f8d6d090a278dee7d86c