[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.
Hey, I (inspired by @pdf) introduced the maximum in the json with this PR. There is no need to stick with it, because it never was in master and its presence has no real advantages, but is hindering introduction of new exitcodes, regardless how often we do this. Everyone introducing a new exitcode must remember to adapt json schema and to bump api version. This is error prone and can easily avoided. Thus, I will remove the maximum, keep the minimum.