some screen locker visual fix

1. string in qml doesn't have "empty" property, use length to check it's
empty or not.
2. AccelButton's string doesn't have &, which will result in unpressable
accel key when process i18n string.
3. AccelButton will display & if showAccel is false, here we introduce a
new property for normal string
4. fallback to loginName if fullName is empty.
5. console.log() in AccelButton is removed.

REVIEW: 108559