[coap] misc enhancements (#5482)

This commit contains a group of smaller changes/renames in `Coap`
modules:
- Add enumeration (c++ style) constants for CoAP `Type` and `Code`
  and `OptionType`.
- Add helpers `Message::Is{Method}Request()` e.g., `IsPostRequest()`
- Define Thread URI Paths (in `uri_paths.hpp/cpp`) as char array.
- Add helper method in `Coap::Message` for common initializations
  e.g., `InitAsConfirmablePost()`.
- Add helper methods for common `Coap:Message` type and code checks
  e.g., `IsConfirmablePost()`.
- Re-define `OT_COAP_TYPE` values to use 2-bit unsigned int values
  (per RFC-7252).

Android.mk

@@ -275,6 +275,7 @@ LOCAL_SRC_FILES                                          := \
     src/core/thread/thread_netif.cpp                        \
     src/core/thread/tmf.cpp                                 \
     src/core/thread/topology.cpp                            \
+    src/core/thread/uri_paths.cpp                           \
     src/core/utils/channel_manager.cpp                      \
     src/core/utils/channel_monitor.cpp                      \
     src/core/utils/child_supervision.cpp                    \