Rename Gantry flag for fullscreen magnifier + mouse bugfix

Fullscreen magnifier moves with mouse is now a bugfix instead
of a feature, so we must rename the flag.

Bug: 354696546
Test: Existing
Flag: com.server.accessibility.enable_magnification_follows_mouse
Change-Id: I3d8289396fa4e951238cf64d503ce37e5a0f9e1f