PowerManager: don't pass NULL pointers to power HAL (2)

The framework can now send the expected duration of the user
interaction along with POWER_HINT_INTERACTION, or zero if unknown.

Similarily to commit ecb6fdda ("PowerManager: don't pass NULL
pointers to power HAL") pass a pointer to a variabile of null value
rather than a NULL pointer.

Change-Id: I019ef3c910d9b501a77e7c03e34841f9d742f0ce