Remove interfaces for namespaces used in native code.

Migrate them to string constants defined directly in DeviceConfig
instead of interfaces. None of these required property names to be
defined in DeviceConfig (or Java code at all) so we don't need to touch
any of the code used by clients of Device Config to migrate these
particular namespaces.

Bug: 126411407
Test: atest FrameworksCoreTests:DeviceConfigTest
Change-Id: I66cabbce23b9765f4ce78bd33b8a08fe07720319