Basic implementation for notifications on lockscreen.

This change makes PhoneStatusBar expand while the lockscreen is
showing. Further, the KeyguardSimpleHostView is shown by
KeyguardBouncer, and the Bouncer is shown whenever the user tries to
unlock the phone (closing the shade).

Bug: 13635952
Change-Id: I4354b7a7937af6e1c93abf16ad6e485376d16bc2