Fix swipe for notification default should be on instead of off

Set default "ON" when we can not get
SWIPE_BOTTOM_TO_NOTIFICATION_ENABLED value.

Bug: 175084985

Test: manual check Settings > System > Gesture > Swipe for notifications
Test: manual check it is disabled after One-handed mode toggled on
Test: make RunSettingsRoboTests ROBOTEST_FILTER=\
"SwipeBottomToNotificationSettingsTest"
Test: make RunSettingsRoboTests ROBOTEST_FILTER=\
"SwipeBottomToNotificationPreferenceControllerTest"

Change-Id: I176ba1c0e18c5ed7d1582d380db6ec190b6e1dec