Only wakeup device for dismiss keyguard when dream activity is on top.

Regression from I62be9283a1d22119eceae5585960b5775a019153.
When dismiss keyguard from shell command, the wakeup signal should only
be used for leaving the dream activity.

Bug: 219376804
Test: atest KeyguardTests KeyguardLockedTests
Test: atest WindowManagerServiceTests
Test: Enable dream, set crediental, wait for dream activity resumed and
keyguard locked. Dismiss keyguard by shell command "dismiss-keyguard",
verify the DreamActivity can be stopped after enter crediental.
Test: Test with test app, verify the device won't wakeup after screen
off.

Change-Id: I4edab8588421b3e341cf3bde07e989ff5e651cfe