subsys/tracing: Use z_current_get in switch_{in,out}

The thread switching tracing hooks are called in the middle of swapping,
before z_thread_entry has a chance to set the z_tls_current value, so they
cannot use k_current_get. Switch them to z_current_get instead.

Signed-off-by: Keith Packard <keithp@keithp.com>