[Tizen][Runtime] Fix using the wrong attribute name for hardware key setting.

In Tizen, the <tizen:setting hwkey-event="disable" /> setting could
disable the hardware key event. But the implementation use the wrong
attribute name "hwkey", need to change it to "hwkey-event" according to
Tizen core spec feature XWALK-1491.

BUG=XWALK-1611
Merge pull request #1892 from takethathe/hwkey-event

[Tizen][Runtime] Fix using the wrong attribute name for hardware key setting.