Do not cancel quick settings expansion onQsIntercept(down)

ACTION_DOWN won't be followed by ACTION_MOVE for qs button tap so
- we should not assume DOWN is a qs expansion event
- we should not call #onQsExpansionStarted on DOWN, which cancels both
qs expansion and notifications animation downwards.

Fixes: 187685856
Test: swipe down from (top of lockscreen|open shade)
      tap qs button before qs expansion ends
      => qs expansion and notifications animation *not* frozen
         and complete running as expected

Change-Id: Ib4f3607b4b8bf9abaebcdf1bdaa09e45abe51297