Poke interactive hint from userActivity and add @SystemApi.

Previously we only poked the interactive hint for input events that
were dispatched through the regular input system.  However, when
using a car dock in projection mode input events are directly
delivered to apps by other system components, bypassing user activity.
So now we poke the interactive hint on all user activity.

Added an @SystemApi version of userActivity.

Added a new signature|system permission called USER_ACTIVITY
to allow system apps to poke userActivity.  For now it seems
prudent to keep DEVICE_POWER signature only.

Bug: 17043684
Change-Id: Ia84bd03083065b8938a9853c08901fbf71b2e56b