This commit updates gRPC core to use io.opencensus:opencensus-api and
io.opencensus:opencensus-contrib-grpc-metrics instead of
com.google.instrumentation:instrumentation-api for stats and tagging. The gRPC
Monitoring Service continues to use instrumentation-api.

The main changes affecting gRPC:

- The StatsContextFactory is replaced by three objects, StatsRecorder, Tagger,
  and TagContextBinarySerializer.

- The StatsRecorder, Tagger, and TagContextBinarySerializer are never null,
  but the objects are no-ops when the OpenCensus implementation is not
  available.

This commit includes changes written by @songy23 and @sebright.

gRPC shouldn't use the state to determine whether to initialize the
CensusStatsModule, because the state can be changed at runtime.

This commit adds a method to set the CensusStatsModule on
AbstractServerImplBuilder and AbstractManagedChannelImplBuilder.  It is simpler
to set the whole CensusStatsModule than pass in all necessary OpenCensus
implementation objects.