Enable Dynamic Button Behaviors for Power/Stem Keys

This change adds new Global Settings keys that control the behavior of
different power and stem primary button presses. These values are read
and used by the PhoneWindowManager, and system components can
dynamically update the behaviors of these button presses by updating the
Global Settings values.

Bug: 283029769
Test: manual
Change-Id: I274a2659fa3fb356a2ae952993dda5b9856b8230