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>
diff --git a/kernel/events.c b/kernel/events.c
index 18ab465..a4fcf17 100644
--- a/kernel/events.c
+++ b/kernel/events.c
@@ -248,7 +248,7 @@
 	}
 
 	wait_condition = options & K_EVENT_WAIT_MASK;
-	thread = z_current_get();
+	thread = k_sched_current_thread_query();
 
 	k_spinlock_key_t  key = k_spin_lock(&event->lock);