Move namespace/prefix logic to Settings.Config.

Previously DeviceConfig, Settings.Config, and SettingsProvider needed to
understand how to append "/" to a namespace to create the prefix for key
names before saving them in SettingsProvider. This removes that
duplication and lets all of the compositing and decompositing happen in
Settings.Config.

Test: atest FrameworksCoreTests:DeviceConfigTest
      atest FrameworksCoreTests:SettingsProviderTest
      atest SettingsProviderTest:DeviceConfigServiceTest
Bug: 136135417

Change-Id: I126155594f3e764e149a4928d1b3708da68c97f4