Ignore ACTION_DOWN events when mTracking is true

This is a partial fix for b/198553252 that lessens the effects and
prevents possible side effects but does not address the root cause,
which is the fact that these events are ACTION_DOWN instead of
ACTION_POINTER_DOWN. The second finger touches do cause the height
of the shade to jump, which is not ideal, but it was unanimously
preferred over dismissing the shade when I polled the team.

Test: manual and atest
Bug: 198553252
Change-Id: I8b087bd435622dcc66e98c731cee1558272be515