Do not use QMediaResource in QMediaContent

Prevents
src/multimedia/playback/qmediacontent.cpp:286:30: warning: ‘QMediaResource QMediaContent::canonicalResource() const’ is deprecated [-Wdeprecated-declarations]
src/multimedia/playback/qmediacontent.cpp:299:30: warning: ‘QMediaResource QMediaContent::canonicalResource() const’ is deprecated [-Wdeprecated-declarations]

Change-Id: Ifd112f38dc300ea83785d52eac83a40cda2b492d
Reviewed-by: Andy Shaw <andy.shaw@qt.io>