Handle down touch events in the home gesture area on locked shade

Touches on the home gesture handle at the bottom of the shade need to
bypass the mUseExternalTouch check, or the shade will only collapse
if notifications are scrolled all the way to the bottom.

Fixes: 407714441
Test: manually ran through shade and unlock CUJs
Flag: NONE bug fix
Change-Id: Iea9fb1717127c1d6f5e3d0b4e224ef87448d51b3