Remove interface for Telephony from DeviceConfig.

Per API council feedback, we are making changes to include only the
namespace in the system API defined in DeviceConfig.java. Strings which
define property names should be defined in code local to the feature
instead.

Bug: 126411407
Test: atest FrameworksCoreTests:DeviceConfigTest
Change-Id: I5961a060201214841f5f0414abc00c33c0c2cbac