Add a config for "wake screen on notifications".

This config value controls whether the "wake screen on notifications"
setting and functionality are available on a given device. By default
the value is "true", which means that there will be no change to any
current functionality.

Test: manually by checking that the "wake screen on notifications"
setting is still present in Settings, and the screen does wake up when a
notification arrives and this setting is enabled.
Bug: 231246919

Change-Id: I37f09fd3bcb820cbfa763bc8aa942924683cf9fb