MagnificationGesturesObserver is used to detect the gestures when
users start to have interaction with the devices through fingers.
If users swipe, single tap or single tap and hold, it means users want
to interact with the UI. If users put two fingers down, it means users
are going to do gestures for magnification operation.
Test: MagnificationGesturesObserverTest
Bug: 149269335
Change-Id: I34ff0694b82ff76c462007f74938e79272d667b4
43988175d2