Fix pinch zoom with partial magnification window not very responsive

When two fingers are too close, the android.view.ScaleGestureDetector will end the scale by minSpan value. This is because the size limit of window magnification can easily trigger the end scale, leading to the zoom out having little effect.

Solution is creating a new construct on android.view.ScaleGestureDetector and set the minSpan to zero.In this way, pinch zoom can work completely on the entire magnification window.

Bug: 295327792
Test: manual
Change-Id: I0afb5021221a7bf00cc1a16299a951f76dd62317