Refactor how StatusBarPolicy knows if keyguard is visible.

In the case where the keyguard has never been shown and thus
never been dismissed: StatusBarPolicy thought the keyguard was
still visible.

Instead of StatusBarPolicy keeping its own internal variable
for wether the keyguard is visible, directly use
StatusBarKeyguardViewManager.isShowing()

BUG:28755086
Change-Id: I1b30c0e6985aa7ff35eea41444ab5504c00ff130