Use user activity touch event to reduce latency of UDFPS

- Send USER_ACTIVITY_EVENT_TOUCH at the beginning of recognizing
  a motion event in the sensor area.
- Currently only UDFPS may trigger the event when the device is not
  awake. Then boost the process scheduling group to top-app for
  systemui so it can update the UI faster. Though the boost timeout
  is 2s, it is usually canceled earlier once the keygaurd finishes
  unlocking which will call setHasTopUi(false).

Also remove the outdated comment in PWM#userActivity because the
method runs on the handler of com.android.server.power.Notifier,
which isn't locked outside years.

Bug: 242775562
Test: Enable AOD and UDFPS, lock device. Unlock with fingerprint.
      The trace "setProcessGroup com.android.systemui to 5" should
      appear with ACTION_UDFPS_ILLUMINATE together.

Change-Id: I1a0174e4b106522ada005089618056861e93d375