[time-ticker] adding 'TimeTicker' class (#5379)

This commit adds `TimeTicker` class which emits periodic ticks (with
one second period interval) to a set of registered tick receiver
modules. The `TimeTicker` would invoke `HandleTimeTick()` method
periodically on the registered receiver objects.

The `TimeTicker` model helps simplify the implementation of many
modules (such as `MeshForwarder`, `MleRoutuer`, `AddressResolver`,
`Ip6`, `DuaManager`, etc) which require periodic update/check of some
state (avoid each class using its own timer).

Android.mk

@@ -191,6 +191,7 @@ LOCAL_SRC_FILES                                          := \
     src/core/common/settings.cpp                            \
     src/core/common/string.cpp                              \
     src/core/common/tasklet.cpp                             \
+    src/core/common/time_ticker.cpp                         \
     src/core/common/timer.cpp                               \
     src/core/common/tlvs.cpp                                \
     src/core/common/trickle_timer.cpp                       \