Context in its own package?

Although `io.grpc.Context` is shipped in its own artifact, and non-gRPC users can also depend on it, the package name still causes branding issue (google/instrumentation-java/issues/162), and a practical issue with OSGI (#2727). We may consider moving it into its own package and splitting it completely from gRPC.

We have two questions to answer:
1. What should the new package name be? Probably something starting with `com.google`, but probably not `com.google.common.context` because it collides with a Google's proprietary context class.
2. How to bridge the old `Context` and `Deadline` to the new ones.