Plumbing for screen pinning request from sysui

Removes lock-to-app request dialog from the AM.

Added a showScreenPinningRequest to IStatusBar to handle the
flow from app requesting lock task to showing the dialog.

This CL also allows startLockTaskOnCurrent (system|signature) to
start lock task directly.  (Note: this is the less locked version
that always allows exit through back + recents)

Bug: 16957435
Change-Id: I284918dd5989de6cb2767c2a717529eb5e9c6db4