1. Removed remoting/codec:codec. Now dependent target have to depend
   either on encoder, decoder or both
2. Cleaned up the logic used to include host-specific code that depends
   on the encoder.
3. enable_remoting is set to false when enable_webrtc is false.
4. Added assert_no_deps for Android JNI target to ensure it doesn't
   depend on ffmpeg

Review-Url: https://codereview.chromium.org/2640783004
Cr-Commit-Position: refs/heads/master@{#445461}