[A11y] Do not move magnifier viewport for synthesized touchpad gestures

Ensures magnifier viewport is not moved for gestures synthesized
from gestures on the touchpad. For example, viewport is moved for
touchpad one finger movements but not for two-finger scrolls, which
are still passed to underlying content.

Bug: b/354696546
Test: Manual on device
Flag: com.android.server.accessibility.enable_magnification_follows_mouse
Change-Id: I504a512c94ae7e0411d1612f036fb131755cb0ce