[flexiglass] Fix touch gesture interruption when dragging HUN

Calling HeadsUpManager.unpinAll before passing the drag gesture to flexiglass was causing an issue where flexiglass visibility, which depends on state set by unpinAll, was set to false, causing the drag gesture to be interrupted by an ACTION_CANCEL.

Bug: 357661886
Test: manually verified that touch gesture is no longer canceled when dragging a HUN
Test: manually verified with logging that unpinAll is called once flexiglass receives the touch gesture
Flag: com.android.systemui.scene_container
Change-Id: I83c964ebbf99141b93f2f43a1512048f917dee20