Add INCALL_POWER_BUTTON_BEHAVIOR setting.

This is part 1 of the fix for bug 2364220 "Accessibility improvements for
ending calls".  This change adds a new (hidden) constant
Settings.Secure.INCALL_POWER_BUTTON_BEHAVIOR, for a setting that controls
whether the Power button hangs up the current call, or just turns off the
screen.

Bug: 2364220

Change-Id: I14d4a89b27e99cac51dc16826d217072d999b2d6