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

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.

Why we need this patch when we have bug 1014418? Because reviewer of bug 1014418 refuses to review until bug 989198 lands. And we don't want bug 989198 got backed out because of issues like 1084187. Thus we would like to land this along with bug 989198 with minimal time difference. Bug 989198 first, then this, with time latency as minimal as possible.

https://bugzilla.mozilla.org/show_bug.cgi?id=1091353