Add volume down as a safe mode key for Stingray.

Fixed a race condition during startup.  We need to wait for the input
devices to actually be ready before trying to detect safe mode.

Fixed a problem during safe mode activation where we would try to add
the overlay window but the display was not initialized.  Now we do it
after the system is ready.

Bug: 3134825
Change-Id: I4c043c142ae6bf0d865c79d266d36154eaf00709