Resurrect Channel.id() with global uniqueness

- Fixes #1810
- Add a new interface ChannelId and its default implementation which generates globally unique channel ID.
- Replace AbstractChannel.hashCode with ChannelId.hashCode() and ChannelId.shortValue()
- Add variants of ByteBuf.hexDump() which accept byte[] instead of ByteBuf.
Bring back ChannelGroup.find(id)
Fix a problem where DefaultChannelId prevents Netty 5 from running on Android

- Fixes #2109
- Use reflection to find the current PID
Get the PID properly on Android

- Related: #2109