[low-power] add csl feature for Thread 1.2 (#4557)

This commit implements the CSL feature in Thread 1.2.

- Add macro definitions for low power to control the compiling of
  source code.

- Add data and methods for running CSL in Mac and SubMac. This mainly
  includes setting CSL parameters, starting/stopping CSL, and the
  timer handling process.

- Add otPlatTimeGetAPI and implementation.

- Add CSL transmission implementation. CSL transmission is a new kind
  of transmission, the related definition and implementaion for the
  whole transmitting process is added.

- Add calling of start/stop CSL in certain cases.

- Implement CSL synchronization maintainence. If a CSL cordinator
  didn't get a frame containing CSL IE for CSLTimeout, the CSL
  receiver is regarded as de-synchronized.

- Add Cli interface for using CSL.

- Implement enhanced Ack with IE. The original code can only generate
  auto ack for Imm-Ack. As CSL requires CSL IE included in enhanced
  ack. This PR implements it.

- Add basic functional test for CSL transmission. More tests
  corresponding to test plan would be added later.

Android.mk

@@ -241,6 +241,7 @@ LOCAL_SRC_FILES                                          := \
     src/core/thread/announce_begin_server.cpp               \
     src/core/thread/announce_sender.cpp                     \
     src/core/thread/child_table.cpp                         \
+    src/core/thread/csl_tx_scheduler.cpp                    \
     src/core/thread/discover_scanner.cpp                    \
     src/core/thread/dua_manager.cpp                         \
     src/core/thread/energy_scan_server.cpp                  \