Reintroduce button-backlight (and respective inactivity timeout)

The power manager rewrite from Change I1d7a52e98f0449f76d70bf421f6a7f245957d1d7
completely removed support for control of the button backlights, which makes
all capacitive buttons out there stay dark. The commit message in that change
mentions it hasn't been implemented _yet_, so this fix should be temporary
until upstream does their own implementation

[RC: Updated to 5.0]

Change-Id: I6094c446e0b8c23f57d30652a3cbd35dee5e821a