New method showDismissibleKeyguard: shows keyguard without locking

Show a message on the keyguard when the keyguard
is kept unlocked after a fold event. Inform
the user to swipe up to continue.

Respect lockdown and biometric lockouts. Don't allow
the keyguard to show unlocked in these cases.

This feature is guarded behind the fold grace period flag.

Bug: 308417021
Flag: ACONFIG com.android.internal.foldables.flags.fold_grace_period_enabled DEVELOPMENT
Test: atest KeyguardViewMediatorTest KeyguardUpdateMonitorTest
Test: manual (adb root && adb shell setprop persist.fold_grace_period_enabled true && adb reboot)
Change-Id: I70a39932cf2e9097df7267c2c6b73e7323de73be