RegisteredServicesCache now saves files to a user-specific dir

Previously, its state was saved in a single xml file inside
/data/system/registered_services.

Services are now stored in separate files stored in a user-specific dir. A
one-off migration step was added, which runs, when the system discovers a
file in the old format.

Bug:19321135
Change-Id: Ie92be01047f4855d01352b6a304820b37344ae2c