Fix lockscreen bugs with SHOW_WHEN_LOCKED

When unlocking to a SHOW_WHEN_LOCKED activity by clicking
a notification, we don't get a the startKeyguardExitAnimation
because Keyguard is not actually going away. Instead, we start
it ourselves.

Also fixes the bug where restarting Keyguard while it's occluded
lead to an inconsistent state where the user got stuck on the
lockscreen.

Bug: 16481924
Change-Id: Iebc5bdd1287bbb81415ae01c7428850acc16c53f