[CS] Remove CS#isKeyguardShowing from SystemActions.

Bug: 277764509
Test: verified via logging that the DISMISS_NOTIFICATION_SHADE
accessibility action is never registered while on lockscreen (even if
the shade is open). Verified that it *is* registered when the shade is
open while unlocked, and unregistered when the shade is closed.
Test: atest SystemActionsTest

Change-Id: If47c11388291f725efa446db6b75e43af8d466de