[cec] Fixed PeripheralCecAdapter application exit code handling.

In CPeripheralCecAdapter::Announce, getting the app exit code from the CVariant "data" paramter did not work as the variant is a plain integer and does not contain a field "shutdown" (https://github.com/xbmc/xbmc/blob/master/xbmc/Application.cpp#L3484). This led to m_iExitCode always being EXITCODE_QUIT.

Consequence was, that on reboot or restart of kodi, TV (and AVR) got switched off, which is not intended (https://github.com/xbmc/xbmc/blob/master/xbmc/peripherals/devices/PeripheralCecAdapter.cpp#L397).

I suggest to include this fix in Helix.
I will remove the "maximum" value completely. On second thought, new exitcodes might be introduced at any point in time. An enum would always and a maximum value might lead to an API change in this case. Not good.