SystemUI: fix up initial cursor down flags on nav bar

We were using the regular flags for the initial ACTION_DOWN event so the
FLAG_KEEP_TOUCH_MODE was not being passed and allowed the cursor to jump
out of bounds slightly.

Change-Id: I5b4d2a5e5e0ce562985fcc3b7b6fac8b4abc49da
Signed-off-by: Roman Birg <roman@cyngn.com>