[posix-app] adopt new HDLC buffer models (#3371)

This commit updates the `HdlcInterface` and `RadioSpinel` to use the
new HDLC buffer model. In particular, for received spinel frames the
decoder uses an `Hdlc::MultiFrameBuffer` which is capable of storing
multiple frames in a FIFO queue manner.

With this model, the received and decoded spinel frames are directly
placed in the FIFO buffer. This allows `RadioSpinel` code during
`WaitResponse()` (when waiting for specific Spinel response frame) to
simply save/keep a notification frame in the queue buffer (without
requiring to copy the frame) so that it can read and process the
saved frame later. If a received frame can be processed at the time,
the frame is then simply discarded from the queue buffer.

Android.mk

@@ -205,7 +205,6 @@ LOCAL_SRC_FILES                                          := \
     src/ncp/spinel_decoder.cpp                              \
     src/ncp/spinel_encoder.cpp                              \
     src/posix/platform/alarm.c                              \
-    src/posix/platform/frame_queue.cpp                      \
     src/posix/platform/hdlc_interface.cpp                   \
     src/posix/platform/logging.c                            \
     src/posix/platform/misc.c                               \