Fix race condition that skips corner updates on half swipe

Set mIsSwiping true before calling SwipeHelper#Callback#onBeginDrag;
otherwise onBeginDrag may use an outdated false and mistakenly include
the swiped view as part of a rounded section.

Fixes: 173753021
Bug: 171817112
Test: half swipe first and last notifs in section; corners always update
Change-Id: I115d30b10657b6edbf94d4d444058bdeeaf9000e