Fix inattentive sleep failing to go to sleep while dreaming

Sets the user activity summary to 0 if the attentive timeout is expired.
This leads `canDreamLocked` to return `false`, and `handleSandman` to
end the dream and put the device to sleep.

Bug: 199967105
Bug: 209963168
Bug: 206737976
Test: atest PowerManagerServiceTest
Change-Id: I33320b6e38886b0c58ace33d901fea38f828d2c6