Asynchronously load SliceData from SliceProvider

The slices paradigm is to return a slice as soon as possible
whith whatever information is currently available. Then, load the
longer information in the background and broadcast on the Uri when
the changes are ready.

This CL incorprates a cache system for SettingsSliceProvider to
load that data in the background to solve the issues of multiple
simultaneous callers.

Bug: 67996923
Test: robotests
Change-Id: I0e3f9984181e1c989fed139707cdb27956cf6de6