Make activity manager sleep top activity when dreaming.

Added the concept of a sleep token to represent a demand to sleep.
We grab this token whenever a dream window is visible.

This change also deletes a special condition in shouldSleepLocked()
which would tend to keep activities running when the lock screen
was shown unless they had previously been sleeping.  I believe this
condition is no longer necessary and the policy is much simpler
without it.  Let's see if anything breaks.

NOTE: This can currently only be used within the system server process
but it might make sense to move it to IActivityManager (guarded by
DEVICE_POWER permission) so that keyguard can use it.  If we added
a callback on release to inform the client that the underlying activity
has finished drawing then we could untangle a bunch of keyguard
specific logic.

Bug: 18866521
Change-Id: I84b2118f5b990a71a1647d1cee536ad3d99f3a24