Listen for SEARCH_LONG_PRESS_HOME_ENABLED settings changes

This is in addition to ASSIST_LONG_PRESS_HOME_ENABLED. Which one
is used is determined by whether Launcher has requested to
override LPH.

Bug: 299672653
Bug: 299969793
Test: Manual
Change-Id: I33d8829156b2ae0d2662227e1940f6aef1c74460