SettingsLib: convert raw fields to getters

Exposing getters as @SystemApi, migrate direct raw field
access to getters.

Bug: 144311098
Test: atest SettingsLibTests
Test: make RunSettingsLibRoboTests -j40
Change-Id: I3bcfd58661b5e28a3829e0f15bed384c4c08d7af
5 files changed