Fix Hard to detect multi-finger multi-tap

The double-tap/touch slope is from system's double-tap detector.
This threshold is too narrow for multi-finger multi-tap. It ends up
canceling the detection when the movement exceed the slope.

We adjust the slopes by multiplying the target finger count.
The maxium slope in 4 fingers is around 4.48 mm is smaller than
the swipe threshold (1 cm)
Bug: 158725482
Test: atest GestureManifoldTest
atest TouchExplorerTest
atest AccessibilityGestureDetectorTest

Change-Id: If0927d15d61a9682da67721d0bf0bbbc1045078b