Have backup TouchController of Taskbar All Apps AFV.

BaseDragLayer only lets the top AFV intercpet touch events, otherwise it
moves onto its TouchController instances. Taskbar AllApps still needs to
intercept touch events sometimes when an AFV above it cannot (e.g. a
snackbar). This CL adds a TouchController to TaskbarOverlayDragLayer
that delegates to TaskbarAllAppsSlideInView if it is open.

Test: Manual
Fix: 299165179
Flag: No
Change-Id: I9b73c55f698de5b60b463f91140a6717a7053cd3