There were three separate "announce ticks" entry points exposed for
use by drivers. Unify them to just a single z_clock_announce()
function, making the "final" tick announcement the business of the
driver only, not the kernel.
Note the oddness with "_sys_idle_elapsed_ticks": this was a global
variable exposed by the kernel. But it was never actually used by the
kernel. It was updated and inspected only within the timer drivers,
and only so that it could be passed back to the kernel as the default
(actually hidden) argument to the announce function. Break this false
dependency by putting this variable into each timer driver
individually.
Signed-off-by: Andy Ross <andrew.j.ross@intel.com>