[Catalyst] Introduce SettingsContract file

The new file contains all contract constants for external usages (e.g.
slice, external Get/Set API). Also reuse existing constants in
SettingsSlicesContract.

NO_IFTTT=Catalyst only

Bug: 388061003
Flag: EXEMPT refactor
Test: atest
Change-Id: I449b7b6cf82e1e102f2c96a21835191605f09943