Use bool var judge whether the navi-keys enabled

if device enabled the system navigation keys,the system
will go to readthe Settings.Secure.SYSTEM_NAVIGATION_KEYS_ENABLED
provider while the driverreport KEYCODE_SYSTEM_NAVIGATION_RIGHT
every times,this operation has a serious impact on performance

Change-Id: Ia30d14c136b7798e2997d43bc601cb361c853e0d
Signed-off-by: 周雷 <zhoulei5@xiaomi.com>