Update javadoc of the newly introduced CarrirConfigManager APIs

This CL updates the javadoc of the newly introduced carrier config
subset retrieval APIs to support keys without default values.

For historical reasons, some AOSP public or OEMs/carriers private
carrier config keys didn't provide default values. When retrieving
configs with such key, the current implementation return empty
bundle which may break those user cases.

The right behavior is that keys without default values will be filtered
out, but other valid keys/values will be kept as is in the returned bundle.

Bug: 244087782
Test: atest CarrirConfigManagerTest
Change-Id: I0f5593f8b5ba54bcd679be26a9a8db31ac33b461