I don't know if the Z2 has a customizable quick launch (you know that thing where you swipe up to get to Google Now) as I don't yet have my Z2...but with my HTC One on CM11 I set the left quick launch bubble to "screen off", which allowed me to quickly turn off the screen from within any app. It's by far the best way to turn off your screen.
Not that you care, but my phone history: Samsung SGH-N105, SGH-S105, Sony Ericsson T68i, Nokia 3650, 3220, Motorola SLVR L7, Blackberry Pearl 8100, original iPhone, 3G, 3GS, & 4, Samsung Galaxy SII Skyrocket, SIII, Note 2, HTC One, LG G2 & Pebble watch, Sony Xperia Z2 & LG G Watch (current)