The big keyguard transition refactor (3/n)

Notify activity manager when dreaming showing state changed so
KeyguardController can update the occluded state when the device
is dreaming.

Test: Set dreaming while charging, wait until screen times out,
make sure that dream is occluding Keyguard.
Bug: 32057734
Change-Id: Ied6f485d9b4a1526cb4cd5f0701f86b1ea05830a