kernel: rename z_current_get() to k_sched_current_thread_query()

The original idea of z_current_get() was to be the counterpart
of k_current_get() when thread local variable for current has
not been initialized if TLS is enabled, otherwise they are
the same function. Now since z_current_get() is being used
outside of core kernel, rename it under kernel namespace so
other subsystem can conceptually use them too.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>