modularize the gesture detection mechanism for magnification

Currently more and more views have similar gesture.
Modularize them to reduce duplicate logic.

Bug: 173159759
Test: atest MagnificationModeSwitchTest
      atest MagnificationGestureDetectorTest
      manually test to see if function works well
Change-Id: I2d33322f331ec26e1d91f0c174e73f33a18acb52