[mac] adding sub-MAC layer (#3262)

This commit introduces sub-MAC layer (class `Mac::SubMac`) in
OpenThread implementing a subset of IEEE802.15.4 MAC primitives which
are shared by both MAC layer (in FTD/MTD modes) and Raw Link (Radio
only mode).

The sub-MAC layer handles the following (if not provided by radio
platform):

  - Ack timeout for frame transmission,
  - CSMA backoff logic,
  - Frame re-transmissions,
  - Energy scan on a single channel and RSSI sampling.

It also act as the interface (to radio platform) for setting/getting
radio configurations such as short or extended addresses and PAN Id.

In addition, this commit contains following improvements in `Mac`:
- The implementation of `STAY_AWAKE_BETWEEN_FRAGMENTS` (delay sleep)
  feature is simplified by adding a new state variable tracking if
  sleep is being delayed and canceling sleep delay on start of any
  operation.
- Member variables in `Mac` class are re-ordered to avoid alignment
  gaps.
- `Mac` timers are combined into one which is used for Active Scan,
  Data Poll timeout, and delaying sleep.

Android.mk

@@ -110,6 +110,8 @@ LOCAL_SRC_FILES                                          := \
     src/core/mac/mac.cpp                                    \
     src/core/mac/mac_filter.cpp                             \
     src/core/mac/mac_frame.cpp                              \
+    src/core/mac/sub_mac.cpp                                \
+    src/core/mac/sub_mac_callbacks.cpp                      \
     src/core/meshcop/announce_begin_client.cpp              \
     src/core/meshcop/border_agent.cpp                       \
     src/core/meshcop/commissioner.cpp                       \