Fix touchexploration multi-finger gesture conflict

WindowMagnificationGestureHandler has higher priority to address
motion events. When the user put two fingers down on the screen,
magnification gesture detector will intercept all motion events.
It ends up the user couldn't perform any multi-finger gestures.

To fix it, we make magnification gesture detection more accurate:
1. swiping gesture sucesses only with one finger.
2. Regarding two-finger gesture, only swipe or stay on the screen
over a duration will be recognized.

Bug: 163016948
Test: manually test: enable Talback and perform 3-finger swipe gesture
      atest com.android.server.accessibility.magnification
Change-Id: I310cf6e3fb2cb2b5b6fbc6a0ba9f0aa1d219b4df