Add KB BL user inactivity time to config

Add a config.xml entry for the keyboard backlight inactivity timeout and
remove the 30s constant timeout.

Bug: 374796267
Test: Update the config in device directory to 10s and confirm that
the timout changes
Test: atest InputTests:KeyboardBacklightControllerTests
Flag: EXEMPT refactor

Change-Id: Id20f3eead5872923da5357335bc3d1b8046b07ae