Add isSliceable API to BasePrefController

Only support explicitly approved Settings Slices,
dictated by controllers which return true for the new
method isSliceable.

Updating the supported settings to a whitelist means that
the method to return all available slices must be updated,
and checking slicability when we index slices.

Test: robotests
Change-Id: I85848c2cdf3e151fa94b33dd1dc5c0374ef94b5b
Merged-In: Ib2b9690cdd0036b5cc4a1cb846c52bce7c824ab9
Fixes: 79779103