Bug 1091353 - Home key and Power key should be only handled by System App

Once bug 989198 lands, all app could receive 'keydown' event, which may cause strange behavior until bug 1014418 lands. This patch call `preventDefault()` on receiving `mozbrowserbeforekeydown` and `keydown` event of `Power`, `Exit`, and `Home` key. This patch should be removed when bug 1014418 lands.

https://bugzilla.mozilla.org/show_bug.cgi?id=1091353