Use custom flag for edge long swipe gesture

Previously we were using same flags that'd be used for regular back key
long press thus being unable to determine whether the event was sent
from EdgeBackGestureHandle or not.

Fixes: https://gitlab.com/LineageOS/issues/android/-/issues/4194
Change-Id: I5c4fd455f581ac5c9c5e3a146095be33e82e8d6e