Update power button gesture menu

This renames the menut to fit within other gesture options. It also properly implements subtitle behaviour in gesture menu and adds a hint on how to invoke the power menu on new Pixels.

Bug:191899039
Bug:168176408
Bug:189506870
Test:  Unit tests for package com.android.setttings.gestures.*
       Manually on device

Change-Id: I985a97514ca142d554ab2aafd61a6219cc1a3dd6