[SettingsProvider] set min and max of backing store number

With this change, the number of users supported for caching will be
UserManager.getMaxSupportedUsers() + 2, but limited to be inside the
range of [3, 18].

BUG: 273617810
Test: atest GenerationRegistryTest
Change-Id: I61c477127068528947a93c9992e47ac79194850e